As ex ProtonMail customer I can tell, what I like most about Tutanota:
Fair pricing. I respect my hard earned money. And Tutanota respects my money as well. They use them wisely for development and don't waste on bullshit PR, marketing and NGO donating.
I really like how Tutanota UI i smooth, fast and simple. ProtonMail i trying to pretend GMAIL, but it's slow and complicated.
Tutanota is "privacy-oriented" service. EVERYTHING possible is encrypted. With ProtonMail only mail body is E2E encrypted, and if you want to encrypt contacts additional info (base info like name and email address can't be ecnrypted) you have to pay for this :) ProtonMail does not support U2F.
I don't like ProtonMail CTO, Bart Butler. IMHO he behaves like a child, eg: provoking hackers https://twitter.com/bartcbutler/status/1011822469651419136 and as software developer I don't agree with his point of view about designing "privacy-oriented" service.