In my person opinion, I believe the DFGT is an excellent upgrade from the DFP!
I had a 100% working DFP, however, after trying a DFGT once at a motor show, I couldn't believe how much of an improvement it was over the DFP. Luckily, I stumbled across a 2nd hand (99.95% new) DFGT for AUD$70 and instantly bought it. I haven't looked back since.
I've plugged both my DFP and DFGT into the PS3 for some 2 player action of GT5P and the difference is very noticeable. The grip is much nicer, you don't get the rubber going all sticky and rubbing off, the wheel is a bit bigger and overall feels nicer. Then the biggest difference is the Force Feedback. The DFP was great when it came out, but the DFGT is just so much better. It's smoother, quieter and seems to give a bit more feedback. With the DFP, sometimes you fight the wheel a bit too much, you can only steer so fast with it, whereas with the DFGT you don't get that problem.
In my opinion, if you can afford it, or you come across a bargain, get the DFGT, you won't regret it!