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).