Wow, I looked at the Forza 5 car list, and it looks utterly haphazard the way T10 chose those cars, and so small a list. And the track list is so bloody short too. Forza 4 seems to be a no brainer in comparison.
However, as much as I like F4's car list, GT6's list is so much greater in variety. Let's say that the outright duplicates in GT6 numbers 200, and the similar sports cars that could be passed over in favor of a more definitive racer numbers another 200. That still leaves around 700 distinctive rides between street cars and race cars. If you didn't do a similar reduction in Forza 4's 500 some odd cars, that's still 200 more vehicles to experience. And I like Gran Turismo's variety.
Another thing to consider is that with Kaz shopping for racing leagues, they need to be well represented with a nice car base of makes for them, so more cars are much more desirable than less. Especially if we get all the tools available for race modding and livery editing, league creation and all that, we're going to be much happier with a larger car list.
Forza 5 and Project CARS are nifty and all, but not everyone is ready to chuck a bunch of cars for the sake of a more focused game. Because for me, Forza 5 in particular is cut to the bone, even compared to Forza 4, and P CARS leaves me feeling similarly.