Pros:
GT5 the cars look better (slightly)
GT5 the cars drive more realistically
Forza puts out DLC faster
Forza will probably have more of the cars you want to drive
Cons:
Forza was made with unrealistic physics to make it easier to play for beginners
GT5 has too many of the same car (85 versions of the R34 Skyline... SERIOUSLY!)
Forza tracks are unrealistic in order to make it easier to play for beginners (I mean really, how wide does the Nurburgring have to be!)
GT5 takes to long to come out with DLC and when they do, its just special editions of THE SAME REGULAR CARS!
Forza lacks 3rd Party Steering Wheel support (its either Microsoft or Fanatec, but that's it!)
Forza Steering Wheel is not accurate (your faster using a controller)
In all, its just apples to oranges, I have personally played both and I still perfer GT5. Forza feels like a Need for Speed game sometimes. Either way both still do not have the Honda HSV-010GT! WTF!