I think I'd need some more time to really answer, but on the outset, I find Mailbox.org's interface more intuitive with more settings and generally feels cleaner and more streamlined. Creating aliases and domain aliases in mailbox seems more proton-like in its simplicity.

Tuta I think is more private and secure, but bits of their interface and app need polish. One reason I think Tuta is more secure despite them both touting security and privacy is that Mailbox search works immediately, whereas Tuta requires you to agree to a permission and states it stores everything locally to you so it may take up space. I think Tuta isn't doing any server-side indexing of any kind? Unsure.

edit: Mailbox doesn't have a native app, and Tuta has a native app but I think it's largely a webview. Notifications work OK but you'll click on a notification and then have to wait for the app to actually connect and resync before you can view it.