I've raced pretty much everything on the 360, with practically everything. I didn't like Forza with the Fanantec. Anyone who has raced at a very high level on Forza will know that brake pressures are stupid high on that game (you'll be killed in the braking zones if you run anything uner 125% pressure.) This means, as a driver, you're forced to be so fine with your braking that you can only use your big toe and push it down like half an inch, otherwise you will cause a severe lock up. This type of braking is almost impossible with the Fantatec brakes, because they are so stiff. But braking on that game is just brutal with any equipment, so if I were you I would get the heck away from anything Forza. It's a bad game, and a terrible sim-racer. Not only that, but you can't make many adjustments with your FFB, so that's a down side.
As for Race Pro, now that's a sim right there. I've been on the PC about 5 months now, and still I remember some of the classic races I had on Race Pro. It's not as tough as Race 07, but it is a sim, no questions asked. If you're looking for a fun drive, I'd recommend you pop in your copy of Race Pro.
But to get back to the topic. Forza or GT5? Easily GT5. I've been told by a few of my console budies that it isn't THAT sim, but it at least has potential. Forza 3 has no future.