I used to be a Mullvad customer but switched to Proton because I use all the products on their suite. It makes financial sense to me.
Mullvad, however, has the best VPN experience ever. Faster, more stable and way less Captchas (though I'm not sure that's good?). Plus, I love their bullshit free pricing. It's 5 euros a month regardless if you buy 1 month or 2 years. Can't recommend it enough, even though I'm no longer a customer.