This is arguably the most ridiculous sales question this year.
FFXIII bundle will OWN the Modern Warfare 2 bundle for so many obvious reasons.
Firstly. There is only one mass sales HD console in Japan, and that is the PS3. In America the Xbox 360 is just ONE of TWO mass sales HD consoles, and sales will probably be split quite evenly between the two for the holidays.
Secondly. Final Fantasy XIII is one of the most anticipated games of the generation, if not all time. Modern Warfare 2 is certainly hotly anticipated, but is not even in the same league hype-wise.
Thirdly. Final Fantasy XIII is exclusive in the region in question. Modern Warfare 2 is not exclusive in any region. While exceptions exist, conventional wisdom holds that major exclusives push more consoles than major multi-platform games.
Fourth. This is the biggest one. For the most part, Modern Warfare 2 (on both consoles) is going to shift 90% of it's sales too people that already have a console for Modern Warfare and other games.
Fifth. REALITY. They will make a tiny amount of Modern Warfare 2 bundles relative to the number of FFXIII bundles.