I can’t give you an in-depth comparison, but I have used both Mobian and postmarketOS on the PinePhone. In fact, I have pmOS as my primary OS and Mobian installed as back up right now. When I first got the PinePhone, I ran Mobian testing for a few years. It was a roller coaster, but that was to be expected. Eventually some issue or another drove me to try pmOS and I immediately found it more performant and stable. The difference isn’t anything massive, though, as they run the same software, more or less. One odd thing I found was that with Mobian, my phone would noticeably heat up during calls, but not with pmOS. I don’t know why. I’ve mostly stuck with pmOS since.

So, overall, I found pmOS to be the better experience. I’m not sure if their decision to add systemd to their OS will be a good one, but we’ll see. Linux on phones is still in a flux. I had to give up using the PinePhone though, since the modem would mysteriously just vanish and a lot of smaller issues started adding up. Finally, MMS not supporting my carrier’s APN settings was what forced me to look at other options.

Hope this helps.