I noticed that this is the first time since the release of Project Cars that Assetto Corsa has had more players than pCars on Steam. That may just be down to the update bringing players on, but you never know.
Really like both AC & pCars but cannot see the physics as any better or worse for that matter, just bit different. But everyone has a personal preference to how their race sim feels. I enjoy them equally and really like that I can go to the other when I get enough of one.
As far as I see it, PCars is not even close to AC. Sure, PCars has a lot of options and stuff, has the better career mode, but all the cars seem to me to be pretty much the same.
Since the release of PCars, I probably have 40 hours invested, compared to at least 5x that amount in AC.