FM3 in-game models are not a patch on GT5's, and GT5 had 16 of them to FM3's 8. End of debate on that one. Even the menu high LOD car models of FM3 are not as good as GT5's. There is no discussion that PD are second to none in that department. The PS3 is a more powerful console. There is the fact that FM3's cars can have an endless combination of liveries and aero upgrades combinations, I'm sure that takes it's toll on the XBOX360's resources.
The PS3 is not a more powerful console, it's pretty much on-par with the 360, but it does have it's own technical strengths..
PD are very very skillful, their LOD system is superb, you can go from cars in a distance at very low LOD's, yet as the car gets closer and closer, the LOD goes up and up and up until it's at this insanely high level. I'm deeply impressed by this on a technical level.
The problem comes from how each spends the consoles resource..
I think it's clear that T10's graphics engine pushes a mass of detail in the track/scenery, they could have reduced that easily to dedicate it to getting more cars on screen.
It's the physics/AI and all the other non-visual per car calculations that limit the amount of cars on track for them (BadNed on FC has said this).
I'd say that IMO, both PD and T10 from their outings so far are slightly skewed too far in opposite directions and need to have a more balanced game.
In the case of FM3, I think they could reduce the physics considerably, fake the grip and other things and make it feel more convincing. Being highly technical (supposedly) about their phsyics isn't going to translate into good 'feeling' cars, and I doubt very few people appreciate all the nuances of their physics. Their AI and collision stuff is about OK (could be better, but passable). They could use this for more cars, or higher detailed cars etc..
You do also need time to optimise the graphics engines, pre-rendering parts you can, reducing numbers of cars on certain tracks, or rolling starts to reduce the load etc, and T10 don't seem to want to do this, perhaps it's time, or experience.