CoD4. Mostly because I can play it on my computer, where multiplayer is worth a damn (console FPS makes me sad in my pants).
Also, it was a big change and a successful one, rather than sticking to a formula.
Don't get me wrong, Halo 3 was a big success. Bungie and Microsoft found their target demographic and delivered what they wanted. Halo never turned my crank (neither did CoD, until 4) and so keeping it the same never expanded it into my tastes.
They both have their advantages, but CoD4 fits better into what I like to play:
PC FPS
Mod Tools (forge is a step in the right direction. though accessible, it's limited)
realistic(ish) weapons/settings (there's a lot of argument here, but it is relatively realistic, even for its faults)
And the multiplayer challenge system was enough to make me need a support group and 12-step program.
Overlooking the error in grammar in the topic, I'd say CoD4. All three Halo games have had rather lame stories and repetitive, boring maps for the single player. The multiplayer is alright in Halo 3, I've not tried CoD4's multiplayer myself. I prefer Halo 3 for multiplayer and CoD4 for single-player.
I didn't buy COD 4 on 360, because it doesn't feel like a console game to me. As for Halo 3, which i enjoy everytime I play on Xbox Live as regular online multiplayer or even online coop, it's more of an arcade game, where the story got lost past the first half of the second title. I will get COD 4 soon on PC, when I'll have enough money. I find Halo 3 very enjoyable on Xbox 360 although i'm a huge PC gamer, so that tells you a lot ^^
COD4 wins without doubt. not only is the battle system fresh, it adds the kick needed in the COD series (modern weapons). wel HALO is just kinda dumb..., i think theres a hundred better games than HALO (take gears of war for example)
COD4 all the way. I've been a fan of the Call of Duty series (the Infinity Ward developed ones anyway) since the original back on the PC. But COD4 really gave me an addiction in terms of multiplayer that Halo 3 simply could not.
although, i do know what you mean, there are plenty of arguments for 'fake' amidst CoD4 - impenetrable and immobile aluminum trash cans, for example.
I think what you're getting at is the total of Halo, the exaggerated physics, environments, weapons, and plot is a little too over-the-top. Whereas CoD4 made more of an attempt at simulation.