Well, one is a surveillance company that's always trying to get more data on people. One is a privacy-focused company with no stated interest in that. Both are in surveillance states with cooperation agreements. FastMail has the edge here short- and long-term.

Ideal case is one end-to-end protected in a country with stronger, legal protections for customers. ProtonMail fits that bill but lacks maturity. Some of us want our mail to definitely be delivered with a provider that will stick around long time. FastMail has the edge there over ProtonMail.