I have both and I use both.

The big benefit from mullvad is that you can get anonymous much easier, it’s easier to ditch an account and it’s harder for you to screw it up. People get mad about proton turning over metadata to authorities or suspending account access but they’re required to do that by law if they can identify the accounts. The structure of what proton offers requires that they have some way of verifying who a user is, so if you’re okay with being able to be identified if someone really tries (or doesn’t really try that hard if you give them payment information or something) then proton is fine for you.

Air is a good cheap vpn for piracy. If you wanna take the maybe smarter route of using separate services for your own privacy and for piracy that’s what I’d do.