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 google screws up, they might see a small blip in add revenue before recovering.
If Protonmail screws up, then they'll lose paying customers which is their only source (presumably) of revenue.