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.

I've used Mobian stable and pmOS stable on both the Librem 5 and PinePhone Pro.

I also can't give in an-depth comparison, but apart from the obvious (differences in available packages, versions and OpenRC vs systemd), I found them basically identical. Both had difficulty keeping the modem connected for more than an hour at a time, but otherwise did everything I expected (wifi, text messages, mobile networks, adaptaive apps, cameras, music/podcasts, etc).

For now, I'm sticking with pmOS, as, for the current stable version, Mobian doesn't offer prebuilt Librem 5 images with full disk encryption support (I can't remember why - this is specific to the device, and I'm hoping it will be fixed in the next release).