Halo 3 is more of a competitive game definately (requiring you to actually win to get XP). As you rank up and get better, the game tries to match you with players at your skill level, so the game is never easy when your past the rank of Major. MLG agrees that Halo is better balanced for tournament play than COD4. It is slower paced and more tactical than COD4.
COD4 is pretty much an easy ride the entire way through, no challenge. Fun though. Nothing really bad. The worst thing is that players you have muted in game will be taken off mute in the game lobby, requiring you to mute them from the Gamercard screen, which takes a bit longer. COD4 gameplay wise, is hard to say. Lots of people complaining about noob weapons/perks. And you can level up all the way without actually winning a single round. Definately the easier of the two. Although COD4 is considerably lag free.
I have both. I prefer Halo 3. It has more lasting value because of the challenge. Custom Games and all the nice throw ups like Forge, Theater, File Shares, and the B.net experience just blows COD4 out of the water. Arcade mode and Prestiege was a pretty sad attempt to add replay value, and the best gun in the game doesn't need to be earned (M16).
A big problem with COD4 online is the way parties are organized. There isn't a party leader, making transitioning from gametypes or joining other parties mid-game an event requiring phone calls and multiple invites.
I am stuck with no money with only Call of Duty 4 and Halo 3 to comfort me. I have played them both into the ground being a brigadier in Halo 3 and having all but one golden gun in Call of Duty 4. I need something new. I also own Mass Effect, a very cool game, alhtough it starts slow and the amount of story variation isn't nearly as impressive as KOTOR 2. GRAW 2 is my tactical shooter, another great game, but the multiplayer is limited with its lack of cover system and icky controls.
I am quite the xbox lover but the graphics on PS3 can sway me if they are good enough (I'm looking at you Resistance 2). As you can see, I like to shoot. Alot. Even on computer, Counter Strike: Source and Americas Army keep me awake very late at night. Now for the good stuff.
I considered buying Call of Duty: World at War (let's call if COD5 for now, W.A.W is not a pleasent acronym). However, what stopped me was the fact that, well, its in World War II, and I have already had my fair share of that (thank you Day of Defeat: Source). By the way, Day of Defeat: Source has the best Kar98k the video game world has ever seen.
One thing that made Modern Warfare great was that it wasn't WWII. You didn't know how the story would turn out, so you paid more attention. IW handled the American and British sides very well and they intertwined perfectly. But the best thing was, you could now get all the modern weapon love that you wanted. Blastin' with an AK is definately way more fun than just that lame ol' M1 Garand. I'ts just more gangsta. Another thing, "aperture sights"? Are you kidding me? C'mon Treyarch, you are ruining my good WWII fun with your plagerism. Infinity Ward should beat your asses. I just saw the video review of COD5 on here (nice one btw) and I have to say: "Is that the exact same game?" I mean seriously! Look! The yellow xp bar at the bottom, the directional pad inventory, the map thing. It all looks pretty much the same, right down to the +10 you see when you get a kill. It really does look like a mod or re-skin.
I'm not saying I don't like WWII games, Brothers in Arms: Hell's Highway looks and plays great. The tactical twist actually keeps me interested rather than just another shooter crazy run around. The gore is awsome too. My carpal tunnel is kicking in so I'm gonna go eat something, later.