I think GT4 will trump Forza in graphics and drivability. GT4 drives more realisticlly thanks to the Pro Force. I think Forza will dominate in car selection (a sore spot in my GT4 heart. The inclusion of 100 Nissans kills you) and the customization is nice, but not required. I think that should be left for street racing. However, I think that if PGR were to do that and take some mechanical features into account it would probably be better the Forza. By that I mean make some street based courses to this game and you'll have a REAL winner. I must say, the street courses in GT4 are a real nice touch. Something no other game until PGR took advantage of. GT this time around has some really nice street courses and I think courses like this (maybe a San Fran course?) would be a perfect addition for Forza. Oh, Forza can also trade PARTS!! not just cars but parts. PD, I am waiting for the online version to see what you have to offer.
Final verdict:(from a diehard GT fan)
Physics: GT4
Graphics: GT4
Cars: Forza
Multiplayer: Duh, Forza
Customization: Yet to be determined, I like tinkering with gear ratios and I don't know if Forza offers that
Replay: Tough to say also. GT4's super realistic feel can make lapping 1 car around a carboard box fun.
Novelity:I'm going to reach out on a limb and say Forza. A new enviornment helps. (GT4 is familar to fans, but new still)
Features: GT, that Photomode sure is fun.
Overall: Yet to be determined, but it WILL be tough.