Mullvad servers tend to get blocked by more sites than Proton so I don't recommend it for installing as an always-on VPN on a firewall for example while Proton is a good choice. It's easier to set up Proton on QubesOS ATM. Mullvad doesn't require any sign-up info like email etc. IVPN has multi-hop VPN routes so it's more private, but will be slower. These are just a few examples.