I've used both for some time and decided to move to Fastmail and so far I am very satisfied.
Fastmail does one thing and does it well, email. There is no PGP support. Protonmail does PGP in the browser via a library that they serve you themselves which I find less than ideal. If you want to do it outside the browser you need to have a subscription and download a bridge software that needs to be running on your machine. With Fastmail if I need PGP in my email I do it via the GPGTools plugin for the Mail app which uses the system gpg installation.
Fastmail does one thing and does it well, email. There is no PGP support. Protonmail does PGP in the browser via a library that they serve you themselves which I find less than ideal. If you want to do it outside the browser you need to have a subscription and download a bridge software that needs to be running on your machine. With Fastmail if I need PGP in my email I do it via the GPGTools plugin for the Mail app which uses the system gpg installation.