I trust the company who is under global regulatory scrutiny 24/7, and watched like hawks by every major news organization in the world for them to slip up.
If Protonmail fucks up, there's not going to be a NYT article about it.
I can't say for sure, but I'd guess that Gmail has many more paying customers than protonmail. This feature exists specifically for those paying (enterprise/gsuite) customers.
If Protonmail fucks up, there's not going to be a NYT article about it.