I've been a longtime Halo fan, and I just recently got into COD. I enjoy both series very much. I think Halo takes a bit more skill since you usually have to aim for the head, and you actually get into tactical battles with people. That being said, Halo matches can often become very silly.
Most people seem to like COD 4 better than World at War, but I don't quite like it as much. The only weapons you really have to use in COD 4 are assault rifles and sniper riles. The LMGs have terrible recoil and an even worse reload time. submachine guns are ok, but you really only need them for close-quarters. Shotguns are downright useless in this game. Sniping takes almost no skill since most of the sniper rifles are semi-automatic, so you can shoot and shoot again without unscoping until you get the kill. Also, a lot of the multiplayer maps seem uncreative. The desert maps all look about the same, really. :/
I believe Call of Duty World at War is better than COD 4 for a variety of reasons. The weapons all seem more balanced to me. Even the starting weapons on multiplayer can have their advantages. Most of the weapons have their strengths and weaknesses varied enough to where you can pick and choose without feeling bound to the same guns everyone else uses (shotguns are actually worth using). The one exception to this would be the MP40, which is generally the n00b weapon. Submachine guns seem dominant in this game, but that's because sniping is all bolt-action, and requires you to make the one-shot-one-kill. Using bolt action rifles without a sniper scope can also be fun. I like the weapon customization and perks in World at War better than 4 as well. The multiplayer maps are also a lot more fun and varied than COD 4's, in my opinion. I also like the WWII theme more than modern warfare, I guess as a history buff. lol
I can't really pick between Halo and COD, but I can say that I like Call of Duty World at War more than I like Call of Duty 4. I am excited about Modern Warfare II, though.