OK, I'm going to try and break things down here, feel free to debate:
FFB: GT5 - Very good, lacking FW feedback. Forza - Fill me in, haven't had enough time on a wheel, but from limited time - Good
Suspension - GT5 - Poor/Average, Forza3 - Average/Good
Brakes (no abs) - GT5 - Average/Good, Forza 3 - Average/Good
Tyres lateral- GT5 - Average, Forza 3 - Good
Tyres longitudinal - GT5 - Good, Forza 3 - Good
Tyres dynamics - GT5 - Poor, Forza 3 - Good
Weight Transfer - GT5 - Good, Forza 3 - Good
Collisions - GT5 - Poor, Forza 3 - good
Visual damage - GT5 - average, Forza 3 - Good
Mechanical damage - GT5 - Very Good, Forza 3 - Excellent
Track accuracy/detail - GT5 - Poor/Excellent, Forza 3 - Average/Very good
Upgrades - GT5 - Average, Forza 3 - Very good
Tuning - GT5 - Average, Forza 3 - Very good
Input response - GT5 - Excellent, Forza 3 - Very good
Car models - GT5 - Very Poor/Excellent, Forza 3 - Good/Very good
Career - GT5 - Average, Forza 3 - Good
DLC - GT5 - Very poor, Forza 3 - Excellent
Playability for noobs - GT5 - Excellent, Forza 3 - Very good
UI - GT5 - Average, Forza 3 - Very good
'Feel' - GT5 - Excellent, Forza 3 - Very good (generalisation, not my opinion here)
Car selection & variety - GT5 - Very Good, Forza 3 - Very good
Graphics - GT5 - Poor/Excellent, Forza 3 - Average/Very good
Haha, the word good suddenly doesn't look at all right to me

My opinion is not final on these, but just a rough idea of my current thoughts.