If your organization already pays for Microsoft 365, use Teams — it's included, and adding Slack on top is redundant spend. If you're not in the Microsoft ecosystem, Slack is the stronger default for teams under 50 people at ~$7.25/seat/mo. The biggest trap: teams that try to run both almost always end up with one tool ignored while paying for both.

Quick Picks (TL;DR)

  • Best for small, fast-moving teams: Slack — channel-first, integrations-first, low overhead
  • Best for Microsoft-heavy organizations: Teams — deep Office 365 integration, structured video calls, shared docs all in one place
  • Best free plan: Teams wins on volume (unlimited chat + calls); Slack's free tier caps message history at 90 days
  • Best external collaboration: Slack Connect lets guests join channels cleanly; Teams guest access requires more onboarding steps
Tool Best for Free plan Starting price Standout feature
Slack Startups, agencies, remote-first teams Yes (90-day history) ~$7.25/seat/mo 2,600+ integrations, Huddles, Slack Connect
Microsoft Teams Office 365 shops, enterprises, hybrid orgs Yes (unlimited chat + calls) ~$6/seat/mo via M365 Native Office co-authoring, Teams Rooms

Slack: Built for Speed, Not Scale

Slack's channel structure — one topic per channel, threaded replies, fully searchable history — keeps communication contextual for distributed teams. It's optimized for keyboard power users: fast navigation, snappy search across years of messages, and an integration ecosystem that connects almost any SaaS tool in minutes.

Best for: Startups, agencies, and remote-first teams that run a mix of tools — developers on GitHub, designers in Figma, ops teams in Notion or Jira — and need everything surfaced in one stream.

Pros

  • Over 2,600 app integrations; connecting GitHub, Jira, Notion, PagerDuty, or Stripe takes minutes via the app directory
  • Huddles (lightweight voice/video rooms) spin up instantly for quick syncs without scheduling a formal meeting
  • Slack Connect shares channels directly with external clients or partners — no Microsoft account required on their end
  • Thread replies keep long discussions off the main channel without losing context
  • Search retrieves messages, files, and links across your entire history on paid plans

Cons

  • Free plan message history is capped at 90 days — conversations older than that vanish unless you upgrade
  • Channel sprawl and notification overload become real problems as headcount grows past 30–40 people without strict naming conventions
  • Video calls inside Slack handle small groups fine but are not suited for 50-person all-hands sessions
  • Per-seat pricing scales steeply: Slack Pro is ~$7.25/seat/mo, Business+ is ~$12.50/seat/mo
  • No native document collaboration — Google Docs or Notion still needs to live alongside it

Who should skip it: Teams already paying for Microsoft 365 who rely daily on SharePoint, Outlook, and OneNote. The integrations Slack offers for those tools are functional workarounds, not native experiences — and you'd be paying twice for messaging.

Microsoft Teams: The Right Default If You're Already on M365

For organizations embedded in the Microsoft 365 ecosystem, Teams functions less like a chat app and more like a unified workspace. Meeting recordings land automatically in SharePoint, documents open for co-editing without leaving the channel, and IT departments get admin controls built for enterprise governance from day one.

Best for: Professional services firms, healthcare, education, government, and any organization where Outlook is the source of truth — particularly those already on Microsoft 365 Business or Enterprise plans.

Pros

  • Included in Microsoft 365 Business subscriptions — zero additional per-seat cost if M365 is already in your budget
  • Native co-authoring of Word, Excel, and PowerPoint documents inside channels, no tab-switching required
  • Teams Meetings and Teams Rooms are purpose-built for structured large-scale calls and conference room hardware setups
  • Compliance, data governance, and audit trail features that enterprise IT and legal teams require
  • Free plan provides unlimited chat and calling — the most generous free tier for straightforward communication needs
  • SharePoint integration means file storage, version control, and permissions are handled without a separate tool

Cons

  • The interface carries more complexity than Slack — reaching a specific setting or conversation takes more clicks
  • Channel and team naming can become a governance problem at scale without a written policy enforced from day one
  • Guest access is functional but cumbersome: external users need either a Microsoft account or a one-time passcode workflow
  • Threaded replies in channels are less intuitive for users arriving from Slack or email-first environments
  • Notifications require deliberate configuration; default settings will ping users for nearly everything

Who should skip it: Small creative teams, freelancers, or early-stage startups with no other Microsoft product in the stack. The interface is engineered for enterprise scale, and much of it goes unused outside that context.

Head-to-Head: Where the Decision Actually Lands

Day-to-Day Messaging

Slack is faster to navigate and built for high-volume async messaging. The UI rewards keyboard shortcuts, and channel switching is deliberately snappy. Teams carries interface weight that reflects its enterprise scope — more powerful in aggregate, but with measurably more friction for rapid back-and-forth.

Video Meetings

Teams wins for structured, large-format calls. Screen sharing plus live document co-editing in the same window is a genuine workflow advantage for all-hands meetings or document review sessions with 50+ attendees. For a 3-person standup, Slack Huddles are faster to open than scheduling a Teams meeting.

Document Collaboration

Teams wins outright. Editing a Word doc in the same tool where the brief was discussed — without switching tabs — removes a meaningful source of friction. Slack has no native equivalent; it routes to Google Docs or Notion for anything beyond file attachments.

Third-Party Integrations

Slack wins. Its ecosystem is broader, webhook configuration is simpler, and Block Kit gives developers a real framework for building custom bots and notifications. Teams integrations exist but the depth and ease of setup rarely matches Slack's.

Total Cost of Ownership

If Microsoft 365 is already in the budget, Teams costs nothing extra. If it isn't, Slack at ~$7.25/seat/mo is cheaper than purchasing an M365 subscription just to access Teams. The math almost always favors whichever platform already has adjacent infrastructure in place.

External Collaboration

Slack Connect is more polished for agency-client workflows. Adding a client to a shared channel requires no Microsoft account and takes under two minutes to explain. Teams guest access works but introduces more onboarding steps for counterparties unfamiliar with the Microsoft ecosystem.

Decision Checklist

Choose Slack if:

  • Your team has fewer than 50 people and runs a mix of non-Microsoft SaaS tools
  • External client collaboration via shared channels is a regular part of your workflow
  • Developer experience and third-party integrations are a priority
  • You are not paying for Microsoft 365

Choose Microsoft Teams if:

  • You already pay for Microsoft 365 — the case for adding Slack on top is nearly non-existent
  • Your team holds frequent structured video meetings and collaborative document reviews
  • Compliance, data governance, or IT admin controls are requirements, not preferences
  • Your file storage and version control already lives in SharePoint

Common Mistakes

  • Running both simultaneously: Some large enterprises do this intentionally, but most small teams that try it end up with one tool as the real workspace and the other as a graveyard. Pick one.
  • Using Slack on the free plan past 90 days: Teams actively working in Slack lose access to all messages older than 90 days on the free tier. If institutional memory matters, budget for at least Slack Pro before your first 90 days expire.
  • Skipping a Teams channel naming policy: Without a written and enforced naming convention from day one, Teams deployments at 20+ people tend to accumulate duplicate or abandoned channels within months. Set the policy before onboarding.
  • Evaluating Teams without considering M365 cost: Teams itself is inexpensive, but full functionality — advanced meetings, expanded file storage, compliance tools — requires a Microsoft 365 subscription that may or may not already be in the budget.

FAQ

Can I use both Slack and Microsoft Teams? Technically yes. Some large enterprises route certain workflows through each. For teams under 100 people, the split-attention cost almost always outweighs any benefit — one becomes the real tool and the other gets checked once a week. Commit to one.

Is Microsoft Teams really free? The standalone free plan covers unlimited chat and calls, which is genuinely sufficient for small teams with simple needs. Advanced meeting features, file storage beyond 10 GB, and compliance tools require a Microsoft 365 subscription.

Which is better for remote teams? Both handle remote work well. Slack has an edge for async-first cultures spanning multiple time zones; Teams has an edge for remote teams that still hold a high volume of synchronous video meetings and document reviews.

Does Slack integrate with Microsoft Office? Yes — Slack has native integrations for OneDrive, SharePoint, and Outlook. They work, but function as workarounds next to the native experience inside Teams. If Office documents are central to daily work, Teams removes the friction entirely.