There is nothing dead about a topic as juicy as this: Two behemoths set to battle it out,
Forza -vs-
GT4.
"Roboteers... Ahh you ready!"
Anyway, we often are cursed by technology raising the bar. GT4 is brand new, yet the series itself is more than 7 years old. We all know that while Kazanori & company may be
'perfectionists' they are not exactly visionaries who embrace innovation and easily change with the times.
Long since the GT series came on scene in summer of 1998, we have been spoilt for such niceties as 3D rims - such a simple, trivial visual detail really - but we have become accustomed to that and now, fully-realized vehicle models is what we demand in our games.
!!HELLO PolyD!! We like oggling over our rides. I mean come on man, esp. in a mode like Photo Mode: seeing them with paper-thin "rims" is lame. I am still dumbfounded that GT still has that. At the same time, I will concede they nailed the animation; in motion GT4 wheels look like nothing I've seen to date.
Rallisport Challenge 2 of course raised the bar to almost hopeless levels in its environment accomplishments, and neither GT4 nor the coming Forza with its cardboard cut-out tree-lined locales can possibly measure up to the volumetric, interactive, and texture-rich goodness that is RSC2's environments.
All that established... You know, there was one kid on gamefaqs.com recently who really got it right: He said if ever there was a time to drop brand-loyalties and just embrace BOTH, this is it. Both these games are simply must-haves for any self-respecting racing sims freak like myself.
The topic says 'Let the battle begin' but you know what? Prolly there can be no definitive winner for the simple reason that each title has such distinct (and numerous) advantages/features that the other doesn't.
Imo, in this case:
you choose, you lose. 
For me, time to hunker down and enjoy both.
Cheers,
-GNJ-