I think this thread has basically fallen into sniping which is a shame.
The fact of the matter is both Halo and Gran Turismo are big franchises.
In this gen only Halo has had a proper game released. Most of the GT talk is based on how it did on previous gens. Until one is fully released we will not know, it will just be speculation. You can't use that as some kind of defence/attack. But what we can say as 100% fact and can't be dismissed or spun is that THIS GEN has already seen a Halo game sell over 11m units.
Looking at he predictions people are saying 10m for GT5. Around 8m for Halo Reach. I think that puts massive pressure on GT5 to get off to a good start. GT4 sold 3.5m in it's first two weeks. To put that into context that is basically what CODMW2 sold in the sametime. It is more then what GTA4 sold. That is the bar this gen and if GT5 is to get to those numbers it needs to beat them. Basically doubling the opening of the previous PS3 exclusive. That is some high hopes!