I use both.

FastMail:

+ simple, lean, paid email.

+ fast web interface

++ working full text search( not just subject line)

+ No ads.

+ iOS App. Fast and simple.

+ Integrated calendar

- No encryption.

- group calendar has been a pain or non intuitive.

Protonmail:

+ Encrypted

+ Lean, safe web interface

+ IOS App that works.

+ or - Free as in cost.

- - - No Full text search.

- can't really configure your chosen email client to it...

- last time I tried, dragged and dropped images disappeared. May have improved UI now.

I only use protonmail for automated alerts and system originated communication.