I'm interested in both, actually. I really have no idea which one i'd get first. On one hand, CoD has been re-releasing CoD4 (which was excellent) for years now, and here we finally have some innovation in it's formula that I would love to see be done, and supporting that innovation with my money sounds like a good deal to me.
On the other hand, I have every major release in the Halo franchise (Halo 1, 2, 3, and Reach) in my collection, and it'd be sad to leave out 4 for me. Plus, i actually really like what im seeing from it. It almost has a bit more of a Crysis look in my eyes, and that seems great to me. So i'm pretty interested to see what 343 Studios can do with the franchise.
So im not sure, i guess i'll just wait for the reviews to get out, and make up my decision when I know more about them on release.