So as I mentioned before Halo 4 launched in this thread: http://gamrconnect.vgchartz.com/thread.php?id=148751 The two series are actually remarkably close.
The COD series (PS3) is at 51.35m while the Halo series is at 51.60m. A difference of just 250k
This makes it interesting for Halo 4 vs COD: BO2 (PS3).
Halo 3 sold 11.64m
COD: MW3 (PS3) sold 12.50m
Difference: 860k
(860k-250k=610k.)
If we ignore any future releases for the franchises, which is going to sell more? There's been talk of COD slowing down, and Halo 4 perhaps selling even better than Halo 3. Either way, it might actually be pretty close.