It depends how much of a "hardcore" gamer you are, I think that the 360 has other games you might enjoy, like the PGR series or Race Pro, and maybe this is the fanboy inside me, but I believe that TDU2 might be better on 360 than on PS3 (but not as good as the PC) if you fancy being part of the console community (pretty much the best part of the moderators)
The reason I think it'll be better is the fact that TDU1 didn't have a PS3 version (and the PS2 and PSP versions were made by a studio in Aus) and Eden's Alone in the Dark for PS3 came out 5 Months after the other versions, and was only slightly better because they changed certain things which made the PC and 360 versions tank.
There's also other xbox IPs that I hope would make a return, like the legendary Rallisport series.
If you want to step outside the world of Racers, the 360 also has a really impressive library...
In short: If you want Forza, and just Forza, and then you'll stop playing the 360 once GT5 comes out, then no, GT5 is only coming out in October, wait...
If you want a 360, with the benefit of playing forza then I reckon it could be worthwhile, because I've always considered the 360 better for racers, particularly because of the controller (although I use a rather knackered 360 FFB wheel...)