There's really no way to know which franchise is bigger at this point. COD4 has considerably more sales when taking all platforms into account(everyone is forgetting that there are an unknown amoutn of PC sales too) but Halo 3 has outsold COD4 on 360. To give Halo 3 the advantage because it's exclusive is a bit odd. No one knows what the effect on sales would have been had COD4 been exclusive to 360 and Halo 3 was the multiplat. It's not a simple "more potential platforms means more potential sales". There is a built in mentality amongst consumers that exclusive titles demand their attention. You can argue against that, but sales history shows exactly that. Show me one multiplat leading the sales of any given console. With all that extra sales potential, surely multiplatform titles would be at the top.
To the OP, both titles will be absolutely fine. There is no cannibalizing environment on Xbox 360. Fable II and Fallout 3 co-existed. Halo 3 and COD 4 co-existed. COD 5 and Gears 2 co-existed. There is always room for games that appeal to similiar demographics to sell.