Tutanota is mainly an email service, with very basic calendar and contact functionality. Proton has a suite of apps, including Mail, Drive, VPN, Calendar, and Pass. I got the Tutanota subscription when it was €1/month and it works well for me.
I used to use both mail mobile apps, but found out that Proton Mail notification doesn't work without Google Play Services, which is a deal breaker for me. So I use Tutanota as my main email and Proton as my backup and for its Drive. Either way, I'm just glad to leave Gmail.
Thank you. I've looked into it, but I didn't feel comfortable logging in with my Proton credentials.