Give both a shot. Both are the only ones (I know of) having zero storage access as the only option; meaning #e2ee is enforced. You may have mailbox.org as a third one (E2EE must be enabled manually there).
I ended up with Proton as I experienced it far more feature rich, flexible and mature. And the Bridge is a must for my use case. In addition, it builds on PGP which can be used to have E2EE communication with people outside of Proton. (yes, I've tried Mailvelope with Tuta; that does not work at all. And doing it manually with copy/paste and PGP in an ordinary text esitor is a waste of time and also turned out error prone one the receiving end; Tuta mails gets mangled on the way).
But if you're a very lightweight mail user, Tuta might fit your need. I generally think of Tuta more like a messenger service with SMTP transport support.
Also beware, importing mails to Tuta is still not possible (unless that has changed the last months). And exporting mails are also a mess. I have migrated one user from Tuta to Proton, and I had to manually fix mail headers to get them imported. The mail export was quite poor, tbh. It took me longer than importing a handful of users from a Zimbra server to Proton - using the same Proton Mail Import/Export tool.
Finally, I just want to mention that Tuta is a company with less than 20-30 employees, serving something like 10 million users. Proton is probably closer to 500 employees these days, serving more than 100 million users. So these organisations are quite different. Which also means they have quite different approaches for developing services further and capabilities to handle sudden challenges.