Okay this is how is it.
Being a pretty big fan of the Halo games I got Reach, I played it for a bit and it was honestly fun... for about 4 days. It was nothing like Halo 3. Even ODST was bad.
Now as for Call of Duty, since Call of Duty 4 I played the COD series ALOT, MW2 kinda killed it for me though because it got old quick.
If you liked Call of Duty: World at War then you will LOVE Treyarch's newest masterpiece, Call of Duty: Black Ops
With Wager match multiplayer, 15 prestige modes and the return of the highly anticipated Zombies mode, Black Ops is with out a doubt the best multiplayer game to ever be released. Yes, I understand this is a very bold statement but it is a statement that I fully agree with and stand behind 100%.
Theater mode btw adds that extra reply-ability which allows you to reach into the archive of every game you have EVERY played and show off your game to all your friends.
My choice would be to get Black Ops, and when you do, come play with the big boys, we will guide the way