E24 I can't tell you which one to choose, however I can tell you that the Mullvad app on my device has practically no impact on battery life and I like that it comes with features like DNS blocking, local network sharing, custom DNS support, easy way to switch servers, account info etc. I don't personally see a reason to use another app.
I have heard though that the Wireguard app might have some advantages. I think that several instances of the same Wireguard app can be used with the same Mullvad configuration, which means that Mullvad running on two or more devices/user profiles would only count as one device from Mullvad's POV, which could allow you to use Mullvad on more than 5 actual devices if you really need it. But I'm not sure about this and how it would work.