They do have a calendar, which is something lacking from ProtonMail (although I believe they are working on this). ProtonMail has a free plan and if you only need a single custom domain then it's cheaper than FastMail. FastMail only have a free trial but the standard plan covers multiple domains (you have to pay more for extra domains on ProtonMail).
ProtonMail seems to be focusing on high security and encryption as their value prop. My interest and needs are more in the web interface UX, speed, workflows, customization, Exchange (iPhone push) support.
Edit (numbers):
FastMail = $50/yr (https://www.fastmail.com/pricing/)
Up to 100 domains (https://www.fastmail.com/help/account/limits.html)
They also do DNS and static website hosting (but not with HTTPS)
ProtonMail = $48/yr (https://protonmail.com/pricing)
1 domain, extra domains: $18/yr per domain