Really glad to see PostmarketOS becoming more viable as a daily driver - there are so many devices which should still work well, if not for Google making OS updates dependent on SoC vendors and carriers...
(e: yes I know there's Treble now but you'd really hope "multiple SPoFs who would all rather you bought the new phone" came up earlier as an issue with updating)
Admittedly not familiar with Mobian - I think the biggest differences are that postmarketOS is based on Alpine Linux (and musl in turn) and encourages porting to a wide variety of devices, while Mobian builds on Debian and attempts to provide more complete support for a smaller number of phones
(e: yes I know there's Treble now but you'd really hope "multiple SPoFs who would all rather you bought the new phone" came up earlier as an issue with updating)