Yes, so it's not the game as a whole, like you're trying to paint. It's some cars, like you mention.
That's because GT has an absurd attention to detail, so much so that it faults other area's at the time. It doesn't mean they're exempt from their own modeling issues though, much like Forza here.
The PS3 era, you'd be correct for about, what, 25% of the actual car list considering the vast majority of the 1000 cars they had were all standard vehicles, looking way farther off than anything that Forza has ever had to worry about. So it's odd that you're going to town on some Forza vehicles here, but you're giving a whole generation of absurdly bad vehicles a pass and saying it was still better
I feel I should reiterate too, the models in x360 vs PS3 era weren't vastly superior as a whole - but what was, again, was the lighting and shader models. As it sits, I would argue that this generations Forza models are trumping GT's, not purely because of modeling and polygons, but because of their attention to detail with things like engine bays and many area's of a car that GT just isn't even modeling at all. Like I said, all about trade off's. They both have their weakness, and strengths really, and next gen is going to be closing that gap they have between each other even more as every generation has so far - meaning a good thing for both games.
GT has a better career mode still? At first they didn't even have one, now they haphazardly threw in one that seems to be hit and miss going off impressions on the GT forum. However, I for one am glad they threw it in, because that's something I always had an issue with(as well as customization.) However, you'd be correct that no Forza game comes close to GT4 career, but can't the same be said about every GT game after that at that point too?