Assetto Corsa is like rFactor in that its built to be heavily mod driven. So it's going to have less 'out-of-the-box' content, but will have lots of opportunity to play it how you want to.
Project Cars will have more content out-of-the-box, but will also be locked down so what you get is what you get. So it will be more of a traditional racing game experience.
I also trust Assetto Corsa's physics more. I've seen some sketchy vids of pCars that make me question its physics and while I've seen people defend it, I kind of trust my eye on this sort of thing. I'm sure it will be decent, but I suspect that AC will be the better 'sim' of the two. But its weird, because AC also seems to be intent on incorporating more diverse and mainstream elements like car showcases, drifting modes and drag racing. They are trying to lay a pretty wide and solid groundbase to be built upon by the community and I think that will pay off in the end.
If you're a sim racing fan, they both absolutely have their place and both should be worth your while. pCars looks absolutely ridiculous, if you haven't seen. I got ridiculed for saying it before in another thread, but I think its one of the best looking games out there, period.