GSuite: Best in class spam filtering. Mail clients are excellent. Integration with the rest of the Google ecosystem is wonderful. Google, as an auth provider, is fantastic. Downtime is nearly non-existent.

O365: Enterprise class mail tools, sub-par protection (spam and phishing will get through). Mail clients are fine. Even better if you're a Windows shop. O365, as an auth provider, is not as clean. Not as fault tolerant, but getting much better.