Haven't tried Fastmail, but switched from Protonmail to Tutamail (because of Proton's ceo beein pro Trump). On Tutamail email arrive in an instant (like verify your email, mails etc), on Protonmail there always was a minute or two delay, sometimes longer.

Honestly, just assume email is an inherently weak protocol in regards to privacy and work from there. So I would suggest getting the cheaper one that fulfills your feature needs and work with E2E encryption like OpenPGP (which also has issues, beware!). Some providers offer encrypting incoming emails with your public key. If you want more secure interpersonal communication look elsewhere (e.g. Signal).

I've been using Proton Mail for a while. It works well for me.

I've been using Proton Unlimited for a few years and I'm planning to switch to Fastmail soon.

Mostly because I dislike Proton not supporting the standard client protocols. I know Proton's "zero-knowledge encryption" is the reason why, but that doesn't feel like the most meaningful privacy gain to me considering it's only for the message body and doesn't apply to email metadata. Proton could try collaborating with and extending open standards with the encryption features they need, making it feasible for third-party clients to implement sync without a bridge, but they haven't.

Needing a mail bridge is a moderate annoyance on desktop. But on mobile it means you're basically forced to use their app. At least the Proton Android app is GPL and I haven't had issues with it, but I don't like the lock-in existing at all. Fastmail in contrast has been pushing forward JMAP as an open standard to make mobile sync on third-party clients better than what's possible in IMAP.

I also don't like Proton Unlimited being limited to 3 domains and 15 total addresses (not counting simplelogin). Fastmail has far higher limits there.

Both services seem to use a fair bit of proprietary software server-side but I think Fastmail has more of the important stuff be FOSS including their main imap/caldav/etc server (Cyrus).

I use fastmail for the email alias ability. Proton doesn't have as robust a system for creating lots of different email addresses that all go to the same place. Also proton is only encrypted if both parties use proton so it's security claims are diminished in my eyes. It's better to assume nothing is encrypted than to think everything is and be wrong.

Happy with Fastmail. Proton has always seemed too pseudo edgy to me. Cranemail while US based is a cheaper alternative by good folks.

Fastmail user here. Never been a problem. It works as advertised.

I can't compare to fastmail since I haven't used it, but I've been very happy with proton mail.

I didn’t love the idea of the Proton basket for all my eggs. Besides I already self host alternatives to the whole g-suite, so I didn’t have the need for a drop in replacement. So I went with Fastmail. I am a huge fan of the alias and masked email options, and beyond those it works as expected for an email service. The mobile apps are decent, and I was able to get them on a de-googled android device. No complaints.

Hmmm . . . looks like their Linux client comes by way of Flathub, and I recall not having the greatest experience with Flathub in the past . . . 🤔

Hmm...no Android widgets, which is a shame.