I agree that GT5 will sell more. Reasons:
1) There has been 3 Halo games this gen (including Wars) and no GT games. As history shows the first GT game in each gen (GT & GT3) have outsold their respective sequels.
2) Reach has competition from Black Ops, GT5's biggest competition is probably Hot Pursuit which is a totally different type of racing game.
3) I think GT games have longer legs than Halo games and GT5 will sell well until GT6 comes out.