I have used both. Both are good. Tuta doesn't support pgp as people said, but I think you'll find that the amount of people you will interact with that can and want to use pgp encrypted email is slim.
The way tuta works is you can send and receive regular email. And when you send it encrypted, the recipient gets a regular email that's says something like"you received a confidential email" (you can edit the text). That person then follows a link in the email and you need to provide them with a password (ideally you provide this password out of band... by text or chat or something... but you can of course just send by regular email).
After they log in, they are basically on a limited web interface to tuta where they can only exchange emails with you (but they can see every email between the two of you in their "inbox).
It's a pretty good system. There is also encrypted calendar and contacts. They have webmail of course and also apps. There's a dedicated calendar app.
Mailbox.org is actually more of a full office suite at this point. The web interface isn't as tight and can be confusing. They can handle your pgp keys or you can do it yourself. You need to decide if you care about trusting someone else with your keys. I actually still have my mailbox.org address because I like the domain. It forwards to my tuta email.
Oh yeah, tuta also allows you to use any of a number of their domains or you can bring your own (pricing may vary). They also have aliasing and catch-all addresses for custom domains.
Both are based in Germany for what it's worth. German privacy laws are pretty strict. For any law enforcement to be granted access to any of your stuff there needs to be a court hearing. They have a warrant canary and transparency report here https://tuta.com/blog/transparency-report .
Also, because tuta is end to end encrypted, all they can release is encrypted data. There's is more of an explanation at the bottom of that transparency report post about what can be requested and what data they even have on users. Mailbox.org might have similar policies but I haven't taken the time to find them.
One thing I will note is that tuta has HSTS enabled I believe so if you're behind a corporate firewall that does certificate snooping by way of MITM when you try to access, it won't connect.