My main concern with Proton is that their Terms seems to be too strict and I was reading many reviews that they might suddenly block mailbox access with no reason. There are many threads on Reddit and web. Also, their Terms say they might block access to mailbox account if its accessed by VPN (except their own VPN) which is also a big issue for me as I am using Mullvad VPN: https://proton.me/support/compromised-account-temporarily-locked
There were cases when people complained that their accounts got blocked for no good reason and Proton did not unblock them.
This really held me back as I did not want to end up with a blocked email address for no reason.
Also, as Proton is a popular choice, I personally think it might be more targeted by dodgy people who will aim to attack/hack it hence its more strict.
With Posteo, its much more flexible - you just use your mailbox as normal with no issues. And its cheap too.
I also like when a company concentrates on 1 product only (like Mullvad is only offering VPN or Posteo is only offering Email services) rather than when its offering a number of different services. It kinda puts me back.
The only thing that I don't like about Posteo is their ugly web interface (Proton is much nicer) but I use Thunderbird/FairMail so don't care about it much.
This is my own opinion and arguments so please take it with a grain of salt.