Fastmail's web interface is *so* much more usable than Mail.app on macOS, there's no contest. I will continue to use it even though it does not have an offline capability at present.
I particularly like Fastmail's easy support for aliases, which Mail.app on iOS mostly supports without meaning to and on macOS the support is better ignored. Having Calendar and Contacts integrated into the mail client is definitely a plus too.
I do wish Fastmail would integrate support for Reminders in their client, though. They can be a storage for them, but they are not presented in their interface yet.