I use both. Fastmail is more compatible with clients and has a good per-client security model. However it is based in Australia, which has increasingly insane digital privacy laws.
Proton mail has more advanced cryptographic features and my understanding is that your email is encrypted so they can't offer it up to third parties without your consent (or at least stealing your password.) The usability is not as good: search often does not work very well (I believe this is due to the encrypted nature of it) and there are only a handful of local clients you can use via their bridge. The mobile app is pretty good, the web app is acceptable but clunky.
I use fastmail for day to day stuff I don't care too much about and protonmail for my more important/personal stuff. Both support custom domains so you can move your email to another provider if you get sick of one.
Proton mail has more advanced cryptographic features and my understanding is that your email is encrypted so they can't offer it up to third parties without your consent (or at least stealing your password.) The usability is not as good: search often does not work very well (I believe this is due to the encrypted nature of it) and there are only a handful of local clients you can use via their bridge. The mobile app is pretty good, the web app is acceptable but clunky.
I use fastmail for day to day stuff I don't care too much about and protonmail for my more important/personal stuff. Both support custom domains so you can move your email to another provider if you get sick of one.