[Mik]

For me it's pretty even. They're both extremely well done. Both have rather annoying problems.
COD4: sniper hit detection is weird, inconsistent hit damage (all tested).
Halo 3: really weird rank system (skill, not EXP), also some weird hit detection (all tested).
I know I'm forgetting stuff but that's all I can remember.

So far, COD4 hasn't been challenging enough, veteran campaign was ridiculously easy except for Mile High Club. I've also found multiplayer to be a little easy. Every once in a while I'll find a group of guys who really know what they're doing though, and that's always fun, except for the fact that my team usually doesn't do anything good those games.
It also seems like there's pretty much no communication between teammates. I usually play headquarters, so I'll go in and kill the guys, and try and destroy the headquarters, and I'll say, "hey spread out for the next headquarters", but for some reason they all come anyways, it's like they think they'll get extra points if they're there as well (you only get extra points if you're the first guy at the headquarters and are there 'till the end). I think Prestige Mode is retarded. Gold weapons are also weird, give me a gold M40 or .50 cal instead of a Dragunov. COD4 custom games don't seem to be very popular, as people are too busy ranking up.

Halo 3 has a lot more competition in its games, and to me is USUALLY more fast-paced. I have tons more friends that play that game, and using Forge and playing Custom Games is awesome. Still not a lot of teamwork, but I play with friends and we all know what we're doing in Halo 3. My main problem with Halo 3 is the Melee system, although a recent Bungie.net update has said that they're working with MLG Pros and other players to improve that system and make it better.
UGh, I'll just stop talking for now.