Forza is going to be better...
the only thing GT4 has up on forza, is a car list, but even that isn't very impressive because half of the car list in GT4 is repeats (like already mentioned with the RX7's), i think it is totally pointless of them to do that, but they want to have an insane amount of cars, and that is the only way to do it without taking up a shitload of memory... anyway, forza is supposed to have the most advanced 4 wheel engine to date, better than GT3 AND GT4, plus it boasts a decent car list itself at something like 250, plus the tunability of cars is far superior in forza than in GT4... at this point i think the GT series might start to fizzle out, many games coming out are very comparable to them and are starting to surpass them, at this point the GT series really doesn't have a lot going for it other than an immense following, but i think they are starting to loose their touch, and are looking towards the wrong area's of a car game, a TRUE enthusiast doesn't want 30 variations of a civic or 20 variations of an RX7, we want a well rounded DIVERSIFIED car list, and a good platform for tuning... if GT4 includes the probe GT, mx6, or the protege, i will buy it, simply because its the only game with those cars in them, but if it doesn't, i'm not even going to waste my time...
GT4 is all hype, i'll be suprised if its actually that much better than GT3...