Mullvad is proven. Not that proton is not, but there were a few controversies about their operations.

Mullvad is accepting payments with actual private crypto currencies. Mullvad had authorities visit their operations site, demanding data and left empty handed as they did not have anything to offer. The same cannot be said for proton. I personally like that they do not offer free services and that they are advocating for privacy through ads and foss projects like the mullvad browser.

Proton is only publishing on f-droid, their vpn and recently their pass application. They have yet to provide notification services for de-googled devices after years of community demands. They have opt out telemetry.(except the proton pass through f-droid.) while mullvad does not, correct me if I am wrong on this.

Since you asked about the VPN, everything mullvad is running is on ram so nothing is saved. (I think this is only for their owned servers though not all of them.)

That being said, I use the proton suite as there is no other alternative right now and the casual user in me is satisfied. :)