Most of the threads here are comparing with mostly baseless accusations. The biggest PS3 exclusive so far is MGS4 which stands at 4.81m which is not too far from its highest cousin from PS2, MGS2:Sons of L at 5.58m. You can see a significant drop of around 700k which can be attributed to the smaller PS3 base. The games which have crossed the 10m mark on the PS2 are the GT and GTA series. These are the only games thus it would come as no surprise that GT5 will easily make its most likely higest debut too. Thus it is useless saying that the biggest PS3 exclusive has not pass the 10m mark since they are not even near the 10m mark even during their PS2 era. Take away Halo 3 and Xbox 360 will not have its own exclusive which has passed the 10m mark too. Both are flagship titles and will easily do 10m and more.
Halo is Xbox's flagship and is a FPS which appeals to NA a lot, thus Halo 3 crossin the 10m is not surprising. However Reach is not a main series. Its hype and preorders are nowhere near Halo 3. And Halo has been released on Xbox 360 before thus there are no huge anticipation in waiting for Halo to reach the new console generation as it did with Halo 3. I am not lambasting Halo but from a logical point, it stands that Reach will do fairly well in the millions region but just not as well as GT5.
Similarly, GT is Sony's flagship but it appeals to not just a core demographic but rather a broad range of consumers since it is a racing and simulation genre. GT5 is also the first in the series to be released on the PS3, being in HD and 3D and having the largest base of vehicles and labels yet. And the most valid point, not only NA is huge on GT, but so is Japan and Europe. This is where GT has an advantage over Halo. If you take a look, every new series in the GT on a new Sony platform has performed huge. If it is a 2nd outing on the same platform, sales drops significantly in NA but actually rises in Japan and EU. And you just need to take a look at GT5:P. A prologue version has sold almost 4m excluding digital sales. Having it cross 10m is not even surprising but a given.
Comparing Reach to GT5 is like comparing GT5:P to Halo 3. Get the parallel comparison for those who dont really get it ? Or rather, its like comparing GT PSP to Halo Reach and I am saying GT PSP will sell more than Halo Reach.