I have no idea what Google Workspace provides, but here's my review of Fastmail:
- The web interface is good, the Android app is good. Relatively fast and straightforward to use. Pretty good reliability aside from the occasional DDoS.
- The calendar is functional but kinda lame. I don't think I've ever noticed before an event happens, and trying to open new events has poor UX. Typical Calendar features work fine.
- Haven't really used Contacts but they're all there from my phone.
- There's a Notes feature which I find mostly useless since it doesn't really sync with anything (though supposedly it can work with Apple Notes' proprietary format?).
- They give you dedicated file storage and transfer quota just to serve files on the web (unrelated to mail quotas). It's accessible via normal browser and via WebDAV client. I guess it's handy, I've used it once, but pulling attachments out of emails is almost the same. If you had to send someone a large file and couldn't send it via e-mail, there you go.
- They have all the mail features that I've used in Gmail. They have some "team" features, I suppose they're helpful if you have multiple users.
- You can connect your Google, Yahoo, Outlook, iCloud, and other accounts in Fastmail so you can actually pull mail from all those places. You can even send e-mail through each of those providers too. So this is pretty cool for consolidating accounts.
- They have their own e-mail masking service, and partnered with 1Password so the masked e-mails have matching credentials in your 1Password account.
- The people running it are using really old tech and don't chase after the latest shiny thing, which I appreciate. Everything they do seems sensible. These aren't tech bros gobbling up VC money to become a Unicorn. And they don't seem to use any trackers or ads at all. The only 3rd party domains I see are from Sentry (for debugging app issues).
- My gripes: the Android app doesn't cache mail or calendar, the Calendar is clunky and notifications don't grab my attention, their new UI in beta will probably become more cramped soon.