GT5 is epic. I have played forza 3 as well, got quite far on it. It is a good game but GT5 is so much more of a simulation really, rather than an arcade game.
Now im not saying forza is an arcade type game, but its a lot closer to it that GT5 ever will be.
GT5 is harder, but with that, i always find it so much more satisfying. For example, throwing a 850 bhp R8 V10 around the nurburgring and getting a good time is very hard work, but so satisfying.
I was on forza the other day driving a 962 porsche. I was driving round the nurburgring and it was so easy to get 6.30 laps in. Some corners you could do flat out and to me, it was totally unrealistic!