You'd be mad to dismiss GT5..
FM3 is great, the handling 'nuance' when on the limit is the best I've experienced on a console, the physics on a technical level are superb.
However, FM3 falls down in that some grip levels aren't correct etc for the car, so compared to real life, some cars aren't quite as expected, but at least if you ignore this, the experience is top notch.
GT5P on the other hand is less sophisticated in the physics department IMO, but they did get the grip levels right most of the time etc, so it made for a still entertaining experience that was close enough to real life to not worry. It's main let down for me was the physics don't react to the 'nuances' of control, with weight transfer when coming off the brakes back on the accelerator, the transitions where a bit dumbed down, making it far too predictable and dare I say too consistent.
By the looks of things, GT5 so far seem to have mixed things up, I'm sure it'll have the correct grip levels and differential behaviour for all cars, and it'll just need a few more things being computed in the physics equation and I think it'll be very convincing..