I technically chose ProtonVPN first when I purchased Proton Unlimited during the beginning of my privacy journey. My main desktop OS was Windows then.
I fully moved to Linux about a year and a half ago and used the ProtonVPN Linux GUI app for a few weeks. I then purchased Mullvad for 6 months because I was frustrated with the lack of WireGuard support on the ProtonVPN GUI app and other features that I now take for granted with Mullvad (e.g., split tunneling). Not being comfortable with using a CLI or WireGuard configs also informed this decision.
I have been re-upping my Mullvad account ever since. I was holding out for the possibility of going back to ProtonVPN (and not having to pay for Mullvad separately as a Proton Unlimited user) since they announced the rewrite of their Linux GUI app. However, after I read that their new app still lacks WireGuard support and split tunneling, I immediately re-upped my Mullvad account for 12 months.
As a final note, I chose Mullvad for the poll because its GUI app for Linux is super solid and easy to use. I don’t have any big complaints about it; I would like for the app to run on Wayland natively, but I would categorize that as a small complaint (the Mullvad team is also well aware and working on it).