Well, i'm here just as a commenter... NO X360 or PS3.
I played GT1&2 on the PSX @ the time, many cars, "real"(but limited) features such as many colors, engine improvement or till racing equips depending of the car, but that magnet physics was always bad, mainly when u touch an opponent.
"I" have GT4 but played few times, it's really WonderFun on the PS2, the photo mode and the filters on the replay make it looks better and even realistic looking.
Then after noticing the features and seeing videos, i say:
- GT5 wins on nº of cars and FM3 on customization;
- GT5 Better dashboard angle camera. / FM3 remember Juiced2 such as MC:LA.
- Graphically almost identical, they have different light system, separated,
both are enjoyable, but sometimes FM3 light on the body resembles SimBin Games.