I prefer Mullvad. I've found it a lot more reliable. I was a paying Proton customer but still had connectivity issues a non-negligible number of times, whereas I've literally never had Mullvad be the cause of connection issues in my years of using it. It's great that they take cash and have literally only an account hash associated with your account.

I've also found that Mullvad customer support are responsive, helpful, and know what they're talking about. I've had experiences with Proton's customer support that were ok, but occasionally had the typical customer service hiccups along the lines of being assigned a new support agent who doesn't read back all the conversation (understandable—I had one bug I was dealing with for months) and you have to explain again what the original issue was and what has been done since.

I think both options are perfectly fine, but I definitely prefer Mullvad, and it's what I recommend to people if they ask me to recommend a VPN service.