IMO they're both good options. Afaik, Tuta has an open source backend, unlike Proton. However, Proton has had great recent independent security audits, and court orders against them have shown how little information they keep.

Proton offers more features and services, which also makes it a bit more expensive. I've used and like both, but right now I have a Proton Unlimited subscription because I like their drive, vpn, and password manager.

https://www.privacyguides.org/en/email/