I would have go GT5

 With the technology they have nowadays, the increased physics, lighting, polygon count, etc etc etc etc. And given that Polyphony wants to deliver a techincal marvel. Multiple disks would do more damage than good. Plus it's just a much prettier package :P hell, this game might be on a dual layer BD xD could you immagine a 4 disk GT game ><

 50GBs is enough. Now GT6 might be different and need the 75GB disc =P 

Forza what's that :P well compared to GT5--- it's nothing!

 Yah 50g should be plenty :P 25 I'm not so sure xD I'm just trying to do simple math... 700 cars, 35+ tracks, 60 Fps, and rumored 1080p xD = stupid ammounts of data.

I didn't realize that the thread was GT5 vs Forza 3... Kinda strange comparison, then. I must retract my statements then above since I know nothing about Forza 3 at all.

Polyphony was one of the best developers in terms of being able to wring as much power as they could out of the PS2, despite its difficult-to-program-for nature.  I expect GT5 to be the same, a technical masterpiece that wrings more out of the PS3 hardware than we've seen to date.  It should also be the most accurate driving simulation around.

That said, GT4 had gameplay issues too numerous to list here (and well-enough-known that listing them would be unnecessary).  I'm hoping that Polyphony hired a Gameplay Engineer to make sure the driving experience isn't mired in these same kinds of problems.  Again, not having played Forza, I can't draw a meaningful comparison in terms of the gameplay issues, but from reading user experiences and reviews, it seems Forza 2 was much better than GT4 in this area. 

I think forza does well (for a 360 game, better than pgr4 for sure)but nothing to Gt5

i think GT5 will be but if it plays like GT3 and not like GT4

I think thats true I loved gt1 2 and 3 then I felt weird playing 4