I've used both. If you don't care that Fastmail is Australian and that your emails don't sit encrypted at rest, they come with more features.

ie:

Mobile client that does contacts/calendar/notes/drive

Mobile setup was really easy on iOS with a policy

Drive lets you host a static website or photo gallery

Custom domain catchall

Proton needs a bridge to use any client but theirs. DMARC/SPF/DKIM was a pretty easy setup for both. Proton is missing mobile clients for their new apps (drive/calendar). Its nice they give you another GB of storage a year, a VPN, and a bunch of the nolog/encryption niceties.