I’ve used them both for a substantial amount of time. By stability, I don’t mean that PureOS has no problems. It has many. I mean that the problems change less often. For example, last week I flashed the latest postmarketOS release to an SD card, booted it on the Librem 5, and things seemed to be working well. Try as I might, that was the last time I could boot that installation. Mysteriously, after shutting it down, the pmOS installation was just bricked. That’s the sort of experience that I’m talking about with postmarketOS that happens more rarely with PureOS. Sure, many things don’t work with PureOS, but there aren’t new major issues popping up all the time.
Also, software freedom is core to Purism’s history of marketing, as well as the language in the founding charter of the company. I get the impression that it is important to many Librem 5 customers. It is to me.
But hey, I’ve tried and extensively used postmarketOS, so I’m not saying that’s unreasonable. I’m just laying down a few points of consideration from my perspective.