When comparing Slack vs Google Play Music, the Slant community recommends Slack for most people. In the question“What are the best Material Design apps?” Slack is ranked 12th while Google Play Music is ranked 15th. The most important reason people chose Slack is:
Slack integrates with tools like Trello, GitHub, Dropbox, Mailchimp, and dozens of others, so you can have a centralized event feed of your project right alongside your chat. This is tremendously useful for keeping context with your discussions.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Robust integration with a huge number of tools
Slack integrates with tools like Trello, GitHub, Dropbox, Mailchimp, and dozens of others, so you can have a centralized event feed of your project right alongside your chat. This is tremendously useful for keeping context with your discussions.
Pro Drag & drop files in channels
You can upload a file to any channel over HTTPS simply by dragging and dropping.
Pro @mentions
You can ping people to get their attention even if they are not online by @mentioning them. Slack supports desktop notifications.
Pro Fantastic search functionality
You can deep search messages, files and snippets. Given Slacks integration into many external services, Slack is good enough to act as the central search interface for your entire team.
Pro Very polished user experience
The entire Slack interface is polished and intuitive to use. There are very few bugs or inconsistencies in the UI and it's very fast to use. There is nothing in particular that is new with Slacks implementation of team chat, but the execution of the groups (called channels), search, external service integration and notifications is close to perfect.
Pro Apps for iOS, Android, Windows Phone, Mac OS, Linux, and Windows
Slack has apps for iOS, Android, Windows Phone, Mac OS, Linux, and Windows to give you the full functionality of Slack with some extra features not found on the website on most major platforms.
Pro Supports multiple teams
You can be signed into multiple teams simultaneously and quickly switch between them.
Pro Flexible, granular notification settings
Notifications are handled separately for mobile and the web app. You can receive notifications for all messages, just direct messages, or based on filters, and you can have different settings for different channels: you don't have to get notified every time someone pushed to GitHub or every time someone posts to off-topic chat, unless you want to.
Pro Freemium plan
Free forever, only restriction on searchable message archives, up to 10k of your team’s most recent messages and 10 apps or service integration. Great for trying out first
Pro Slackbot extensible chat robot
The "Slackbot" can is an extensible robot that can be set you remind you about tasks, auto respond to certain phrases and a variety of other functionality.
Pro Syntax coloring
Pasted code can be colored based on syntax.
Pro Inline link previews (photos, mockups, etc.)
When a link is added, some content in the link is shown such as image - like how Facebook does it when you share a link.
Pro Edit messages easily
It allows you to change what you sent by hovering to the message and selecting "Edit message" under the ellipsis (...).
Pro Emoji reactions to limit excessive posts and notifications
Pro Shows local time of each participant
You can click on the profile of a user to see their local time. An especially useful feature when members of your team are working in different timezones.
Pro Dev team is invested, responsive, and friendly
Having submitted both feedback and support requests for bot development, I can personally attest that the team takes feedback seriously, and responds quickly to communication. This is vital for any closed-source or hosted project.
Pro IRC connectivity over SSL
Pro Self chatting
Creative implementation which is very good for personal journaling or drafting/collecting ideas. Previously, to do this, users needed to create a private channel with themselves. Note that some other team-chat apps may disallow creating group/channel with no 2nd person(s).
Pro It can surprise you
There's a checkbox in preferences under advanced options that may surprise you.
Pro Multiple channels for different groups
Pro Upload library through browser
Google has added the option of uploading songs through the browser with the help of a Chrome extension download as well as directly through the desktop Chrome app.
Pro Instant mixes
Google Play Music can make smart recommendations for playlist by the user just picking an artist or song they want recommendations formed from.
Pro Free no advertisements
Google Play Music is a free service for storing and streaming ones library that has zero advertisements in the interface as well as the playing music.
Pro Also offers streaming service
GPM can be used as a normal music player but also as a streaming service like Spotify.
Cons
Con Expensive when you need to upgrade
At $6.67 per user / month (or $8 if billed monthly) , Slack is significantly more expensive than the competition if you need features such as unlimited integrations (more than 10) or unlimited message storage (more than 10,000). However, the free version of Slack includes unlimited users.
However if you need only unlimited messages you can use storage services like https://slarck.com to upload then browse and search your entire message history, while staying in Slack's free plan. So with a combo of Slack+Slarck this con is not that major.
Con No self-hosting available
If you are worried about third-parties getting access to your data you should consider self-hosting. With self-hosting you are in control over where your data is stored, who has access to it. You will also not be vulnerable to exploits of a third-party provider.
Con Hidden max limit of free users per channel
Slack says that their free accounts support an unlimited number of users, which is true. However they don't mention that there is an undisclosed maximum number of users per channel (8462). For a large open source community, this is something to keep in mind.
Con Proprietary (non-free/libre)
Con Linux client is very RAM intensive

Con "Native" desktop apps are web apps
While it's great that Slack provides installable apps for Windows, macOS, and Linux, they're just the Slack web app wrapped in Electron shell. This means they don't offer the same level of native UX that a truly native toolkit app would.
Con Slow and lags sometimes

Con Awful performance and constant glitches, since it is Chromium-based
You will experience a lot of hangs and glitches and it eats immense (for as basic as UI is) amount of RAM.

Con API is overall very poor
Can't do much with integrations.
Con No E2E encryption
Data is sent of SSL only, not E2E encrypted.

Con API doesn't allow custom widgets in chat
Con Replies are the worst feature implementation of all time
Con Status bar not sensitive nor customizable
There is no way to go easily go back a specific amount of time (for example: Audible has a button that allows you to go back 30 seconds).
The status bar could be more time specific. If you want to go back to a particular segment of listening, there is no way to know exactly where you will land. You have to do some kind of guessing and it is annoying. Lot of time wasted in searching in a blind manner.
Con Quality only as high as 320kbps MP3s
There is no option to listen to lossless music, even if that is what the user has uploaded. The highest quality is 320kbps MP3s. If a lossless track has been uploaded and the user then downloads the track at a later date the file will also only be a 320kbps MP3.
Con No way to start the app in ones library
The app offers no way to actually start up in ones library, either in the desktop Chrome app or the Android app. For those that only use this app for listening to their purchased or uploaded music this is pretty annoying as there s no real need for the front page of recommendations and activity. This doe snot necessarily need to be some social platform but just a way to listen to ones music, really the user should have the choice, especially if they are paying for this service through purchased music or their streaming service.