It depends, really.
Let's look at trends.
Halo 3 > Halo 2 > Halo 1. Nice, simple.
GT3>GT1>GT4>GT2. However, GT3 was bundled a fair bit, while GT4 received no bundling at all. Halo 3 will probably overtake GT4 by the end of this gen, even taking into account the fact that it is undertracked (Polyphony stated they'd shipped over 10 million, I doubt there are 410k copies of a five year old game lying around on shelves)
Opening week will definitely go to Halo 3. I can see GT5 doing around 3 million, but won't come near Halo 3's 4 million. Of course, more than 3/4 of that opening week was in America, while GT will be more spread out: I would say a million in America, million and a half in Others, half a million in Japan.
I would say that LT sales will go to Halo 3 as well, but not by much. People throw around GT4P's sales and GT4's sales. Well, GT4P wasn't on the market for nearly as long as GT5P, and it wasn't even released in Europe. GT3 may have launched on a 10 million userbase, but it was on a system with Wii-level weekly sales, and as I mentioned, it was bundled.
GT5 will probably be bundled as well, but the PS3 will only reach around half of the PS2's userbase. It's not simple arithmetic, because attach rates decrease with increasing userbase.
So I would say 10.5 million for Halo 3, 9 million for Gran Turismo 5. Excellent sales, but not Halo level.
If the PS4 is a runaway success, like the Wii or PS2, I could see GT6 cracking 10 million easily. But judging by Polyphony Digital's recent behaviour, we can expect that in around 2021 
(Former) Lead Moderator and (Eternal) VGC Detective 
