Well, Halo (ODST aside) has seen increasing sales while GT dipped a bit. However GT5:P sales where very good - going by VG sales plus allowing for downloads on top just a short taster has sold millions.
So... it's going to be close, but I think LTD GT5 will slightly outsell Reach given it has so much content plus Nascar plus it's the first true release of the franchise on PS3.
Try to be reasonable... its easier than you think...