I also have both (actually I have all simulators except rF2) and can with confidence state that pCars physics are really good. They are somewhat on the conservative side though when it comes to progression of grip (read: some of the cars are quite easy to recover from nasty situations.. others are not!) but the actual amount of stuff and complexity of the simulation is in a league of it's own. Heat transfers from breaks properly to various parts of the tires. The tire model is insanely complex (by far the most complex on the market at this point until iRacing gets their shit together).. for instance flat spots and aquaplaning naturally fall out of the simulation. No "canned" stuff or look-up-table stuff here. It's all dynamic and all tied to the completely dynamic weather/track temperature/moistness. Unfortunately the flat spots got tuned down quite heavily as people were complaining about their wheels shaking like crazy in the single seaters.. I hope they give us some kind of "tire damage" slider and allow for completely realistic settings after the game is released (will not happen at release though.. maybe in a patch if the community is vocal enough of this). At least we have witnessed the full awesomeness of the tire dynamics, it's just been tuned down by too much, in my opinion. It should be possible to completely destroy a slick tire in one single corner if you screw up.. that used to be the case a few hundred builds ago. It's not any more though.
I'd say though that there are still some oddities with perhaps the differential or dampers.. something is still not perfect when it comes to the "motion" of some of the cars. This is where especially AC excells as they bob and weave in a way that I feel is more natural (not as wobbly or odd as iRacing and not as stiff and "rigid" as pCars). However, it's been said several times that this may be a visual thing as well.. not purely physics. I still think AC car cockpits and the way the head movement is done is the most natural looking movement in any game to-date.
FFB is one thing that pCars absolutely kills the competition in my opinion. First of all, it is extremely flexible and you can spend YEARS learning to set it up exactly as you want it (this is a positive for me but can be a negative for some people!). The detail, once setup to your liking, in the FFB is second to none. It's the only time I've played a sim where I can CLEARLY feel the exact optimum slip angle that I should be at which makes me LEARN to be a better driver and not putting too much lock or too little lock into each corner. It also immediately tells you if you've over cooked a corner or if there is more speed to be found in a corner. It's just so ridiculously detailed, but not over the top.
The only thing some people are missing from the FFB is a constant road rumble and "jolting around" as can be found in AC and iRacing but in my opinion these are completely secondary FFB effects and not at all pleasant! They are not at all realistic either.. when was the last time your real car's steering wheel was violently jolting around on a road? Right.. never. Because it doesn't do that. If you jump into a single seater or a stiffly sprung race car then it starts to be somewhat more violent.. even in pCars. Luckily you can tweak even this.. you simply exaggerate Fx (read: Fx TIRE forces, as in Fx, Fy, Fz and Mz tire forces.. not "effects") forces and dial in some aggressive "scoop" in the settings and you can have a shaky, weird and violent wheel too.. if that's your thing. It's also highly track specific. Some tracks are a lot more bumpy than others, which brings me to..
.. tracks. Only a hand full are laser scanned (4 of the British tracks) and it shows, unfortunately. I couldn't give a flying f**k about "accurate bumps" and crap that people always seem to associate with laser scanned tracks, no.. but the ACCURATE cambers of laser scanned tracks just makes for a much better drive. There is no competition. People saying laser scanning is over rated are crazy. Laser scanned tracks are INFINITELY better than the "normal" tracks due to how they "flow" from corner to corner.
Heck I like the laser scanned Nordschleife of AC so much that I would recommend the sim to everybody simply because of this one track alone. The Nords in pCars is not bad at all, probably the best non-laser scanned version ever created, but it's still way off the laser scanned Nords in AC. It's not even remotely in the same league.
Hopefully we get more laser scanned content for pCars in the future.
AI is an area where pCars will be shining (and already is, depending on car+track combo though). It's way better than AC (which is not saying much as the AI in Assetto Corsa is very weak) and I'd even go as far as saying that it's better, or at least "more natural" than in Stock Car Extreme, which held the previous record for great AI. They are fast enough to give even aliens a challenge (without them cheating!) and smart enough to not let you pass too easily into slow corners. They are also not afraid of going past you if you leave the door open but will race pretty clean too without causing a lot of mayhem. In short: Racing the AI in pCars is awesome! Just make sure you treat them like real people and they'll give you an exceptionally good race!
Now lets just hope the online experience will be good as well (I have very little experience with online racing in pCars) and it should be a great title with a long life.