Very interesting thread, i wont say which game will sell more only time can tell that but one important factor that a lot of people keep leaving out when comparing the two is the fact that GT covers all demographics, age, gender etc... While Halo is strictly a "gamer's" game GT covers a way larger demographic, from casual gamers to Sim racing fans, to the hardcore ones that bought/will buy a ps3 just to play GT to car enthusiasts to people that will just buy the game because of how it looks, its just one of those must have titles whether or not you buy it on launch day or 3 years later in the bargain bin. Thats why GT games have such long legs, it has very little to do wtih install base or racing genre trends, so to try to predict how GT will sell based on previous GTs or PS3s install base or on how previous PS3 games have done would be a bit misleading.
That being said both games are flagship titles for their respective consoles and I think both games will sell very well and i think its even more impressive to see Halo sell that much while it covers such a small demographic but like many said in this thread, this gen is a "shooter" gen.
