I tried both. Proton email client on Android at least was awful. Super sluggish to navigate. In fact I have a chunk of credit with them because I cancelled too late to get a refund. No idea what I'm going to do with that. I already have a VPN and a Pwd manager...
Fastmail has been snappy and I like that the app has a notes section for quick jotting of ideas. I also like that Rclone can attach directly to Fastmail files. They just recently added Proton Drive support too though.