I've played all the GT's on the PS1 and PS2, but I prefer the xbox version of the GT series named "Forza Motorsport".
Forza's physics and car customization are much better than the GT series. Also I feel much more comfortable playing Forza than GT. Plus damage in Forza is more significant than in GT. Hit a wall in GT, the car still looks brand new compared to hitting a wall in Forza, your car will sure look ready to go to the junkyard at end of race if you're not careful.
damage is a plus in forza I'll give it that
Forza's car selection isnt as much repetitive as GT's 15+ Skyline GT-R's, 12+ Mitsubishi Lancer Evo's, you name it plus the advantage of Ferrari and Porsche that havent been available in GT (except for the RUF's -modded Porsches)
Drifting in Forza is much easier than in GT as long as the ABS, TCS and yaw controls are turned off.
drifting in gt4 is harder, but more satisfing, because cars in forza natually slides, in fact, they shouldn't be sliding that much.
However I have seen shots of the PS3's GT-HD....all I could say was "Wow" especially cause it now includes both cars and bikes together!
yeah, and that isn't even gt5, it's only a gt4.5, gt5 will be even better
and oh speaking of the Nurburgring Nordschleife, I've played both versions of the track in GT4 and Forza, I prefer Forza as it is more wider than GT's. and GT's start/finish line is different from Forza's start/finish line.
GT's start line was the old starting line, and forza's line was the new one, and also, GT4's nurb is more realistic, that is actually how wide the nurb is, so imagine driving it in real life. forza's one is also smoother, while real nurb is very bumpy and old.