Well after a good 15-20 races on each Dirt and Forza, I gotta say I'm just going to get dirt.
I only race for fun, and I only use a wheel for racing games. I just don't know enough about cars to really get into a few 100 hours of Forza. It's a shame though, there isn't a racing game I've played that compares to Forza when your using the wheel, it controls perfectly.
Dirt not so much, it's more like PGR3...I think those games try to incorporate drifting so much it *****s with the control on the wheel because it's much harder to keep the cars going straight and not fishtailing 80% of the track.
But in the end I like the rally aspect more with the jumps/rollovers/better damage. I spend 1/2 my time in any racing game ramming other cars on purpose, not really the point of Forza...or Dirt for that matter I gues, but DIRT is still just more fun in the end for me.