Protonmail have a free plan, whereas Fastmail only has a 30 day trial.
But at the $5/month level Fastmail appear to be much better value (30GB vs 5GB storage, support multiple custom domains vs single domain, 600 alias addresses vs only 5 at protonmail).
I also like what fastmail are doing with JMAP.
As they're both free for at least 30 days try them both out?
edit: fastmail also has good notes integration with iOS if that's of interest
But at the $5/month level Fastmail appear to be much better value (30GB vs 5GB storage, support multiple custom domains vs single domain, 600 alias addresses vs only 5 at protonmail).
I also like what fastmail are doing with JMAP.
As they're both free for at least 30 days try them both out?
edit: fastmail also has good notes integration with iOS if that's of interest