I agree, it is too early to tell, but let me make this statement:
Halo and Call of Duty both have great series of games, but they have totally different play styles. Yes, they are both FPS games and good ones at that but they are both played differently so its all based on which style you like to play.
I also wanna point out that the Halo fans are going to obviously say Halo, and people who like CoD will say CoD.
In my own opinion, Bungie had me hooked when they released the first Halo, then Halo 2 came out and it was a very fun game but I still love the first one. If they make another pretty game and have new guns or remodel the old ones, either way I think it will be an awesome action-packed game. This also goes for CoD. When the second CoD game came out i loved it. I did not like CoD4 much but i do play it from time to time with friends. I do like how MW2 is looking in the all the videos but if it was up to me to decide between the two right now with the information i know about both series, i would chose halo, but there is a long time before you can actually make the right decision.