Halo 3 would win.
If people can look beyond the biasedness of their fanboyselves (is that a word?) they should look at some facts.
GT4 sold 9m in 5 years with an install base of 120m units. That means just 7% bought that game.
Its first week sales worldwide were 2.4m with again 120m units to sell to.
Halo 3 has sold 9m in 15 months with at that point 18m units to sell to. 50% bought the game.
Its first week sales were 3.8m worldwide, nearly double the opening week of GT4.
By the end of its 10th week on sales Halo 3 sold 5.9m units worldwide.
GT4 sold 5.09m by its 10th week. Meaning Halo 3 outsold GT4 by almost 1m by the end of its 10th week. Halo 3 has over 3 years left of sales before it is has had the same amount of time as GT4 on sale. Meaning Halo 3 sales will obviously increase by a few more million in 3 years. With the expansions coming more will be sold.
Pro-logue has been out a year and has sold nearly 3m. At the end of its 1st week it sold just 600k. Not even comparable.
So there is no question Halo 3 would win because it sold more then the last proper Gran Turismo game with over 100m less units to sell to and with 3 and a half years less time to sell the game.
To assume it will match the sales of A-Spec is laughable.
