I think it will be close. They are both flagship franchises. In favour of Halo is the fact that the main line games have increased in sales with each succeeding game. GT franchise has oscillated up and down, though because of what GT5 is in the scheme of the franchise it should be well up there.
Reach can get the world record for week 1 sales for a console exclusive I think. And it'll keep it for a long time too.
Given the uncertainty with SW sales (greater uncertainty than HW I think) I'm going to say that there won't be a statistically significant difference. Unless of course one considers only the publisher's shipped to retail figures which will be pin point accurate.
My gut says Reach will beat Halo 3, I don't think ODST's sales really indicate any sort of decline. It's an expansion pack and expansions never sell as well as the main game. I'm not sure GT6 is goiong to cross 11 million LTD with the likely LTD PS3 install base of somewhere between 50 and 70 million.
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