Well, there is no real benchmark that works equally well for both consoles, so the only thing that one could go by is the FLOPS rating that Sony and Microsoft gave out, which are probably not very precise. Judging by this, the PS3 is quite a bit more powerful than the Xbox360.
What's worrying me the most, though, is that T10 stated that they had to reduce the numbers of cars on track from 12 to eight during development, due to the Xbox' limitations.
On the other hand, two years of optimizing and polishing can do a lot to a graphics engine and a physics engine as well.
With GT back in the ring, T10 has to bring their a-game, though. Which can only be good for those of us that own both consoles
If they can get the showroom models to work during racing and have them look exactly what they look in the dealership, I'd be fine with copyoasting the cars from FM3. It's not like they're from a last gen game