Just adding to what was already said here..
I personally use ProtonVPN with a paid subscription. It works great for me. I've also used Mullvad a few times.
ProtonVPN is more convenient in some ways than Mullvad. Like, you can have a subscription. With Mullvad, keeping up with payments would be more of a chore I think.
I'd also agree that the ProtonVPN app is more polished, but there are things I'd like to see improved.
Both apps have been audited, as well as server instances. Mullvad has been audited more times (if I remember correctly) and I recall the reports were more in depth, but both have proven they don't keep logs for their paid services. Check out their websites and read through them yourself.
We can always debate which VPN provider is more likely to cave in to government requests. I've read many things about this. I forget where Mullvad is based, but I've read that country is not as good for whatever privacy reason or Proton Mail turned over an IP address for someone, etc. etc. etc.
This part of the debate can go on and on but at this point we don't have any evidence either VPN provider has given data to any government. It's all hypothetical. If you really need true anonymity, use Tor, otherwise, I think you'd be happy with either VPN for normal use.