I wouldn't take back GT5P, but that's just because I love it.
Dirt 2 is obviously more of a full game.
You should try the Dirt 2 demo to see if like it.
If you are looking for a very fun game which will keep you playing for hours then yes. The physics and gameplay are a lot more arcadey in comparison to GT5P but it still works great with a wheel and you should not encounter any problems with your Driving Force GT(Mine works fine anyway).
You can always pick GT5P up again used later, it's cheap to buy pre-owned now.
If you are really unsure, do what Lancia Rally said and try the demo.👍