Comparing games as a whole is a futile exercise IMO, unless of course the games are very similar in scope and design and the end goals are similar. Forza vs. GT is a closer comparison than PCars to either one because they are aiming at different targets. Forza and GT are more of a shotgun approach trying to hit everything in sight and not being too discriminating, and PCars is aiming at a smaller audience that simply enjoys the thrill of driving and racing for it's own sake, without a lot of extras throw in like bigger budget games like Forza and GT tend to do. To succeed, PCars has to be better at it's narrower focus than other games, otherwise it's just a slimmed down version of what we have been playing for years.
I would compare the elements of the games that are directly comparable. Graphics, physics, sounds, FFB, UI etc. and even then it's very user specific. Everyone has different levels of preference and things they are willing to trade off. For me, physics/FFB and sound are the top priorities, graphics should be good but I'm not fussy on them being "perfect", although it's a bonus if they are. I'm fine with trading off graphics for car count for example so if I had a PS4 I'd be happy with the car detail, so long as I can get 30-40 cars on track. Others may not be. Some might think you can have 250k polygons on cars and 45 on track and hold up 60fps in most conditions.
To me there are just too many variables, and too much personal preference to give an objective opinion about one game over the other.
