One of the questions on the minds of Call of Duty: Modern Warfare owners when it comes to Call of Duty: World at War is whether or not its worth the money, especially for those who could care less about the new campaign and will spend most of their time in the multiplayer mode fighting alongside and going at it against their fellow peeps.
While there are many similarities between the two games, there are also some distinct differences. As far as the main story goes, they are both pretty even, each delivering a fantastic story with theater-like moments that really put you into the thick of the action. It's a definite tie on which delivers the better story -- both are fantastic -- and both are probably a little too short.
The first thing to look at is what is new about the game. While there are obvious differences between weapon selection, a few new perks, and dogs of war substituted for calling in a helicopter, there are two really big things new in Call of Duty: World at War.

Vehicles and Vehicle Perks. This is one of the really big additions to some game types like death matches and headquarters matches. The ability to jump in a tank, drive it around and kill the enemy creates an entirely new dynamic. Unfortunately, it can sometimes be annoying. It'd be nice if there were some non-vehicle modes of certain matches like team deathmatch for those that find the idea of getting blown up by a tank more annoying than fun -- but when all is said and done, I do think they add a bit of strategy to some maps -- it really makes you want that sticky bomb over the frag grenade and the bazooka over the bouncing betty perk.
Co-Op Mission Mode. This is definitely the big bonus of World at War for those who like to go up against the AI with a couple of their buddies. In addition to running through missions with your friends, you can also unlock a Nazi zombie mode for a nice little distraction.
Multiplayer Modes. Most of the same modes in Modern Warfare have found their way into World at War, but there are a few modes that have vanished. Mercenary Team Deathmatch, Old School FFA and Cage Match are all missing in action in World at War. While Cage Match being dropped is a little understandable -- though it is fun to go one vs one at times -- and mercenary team deathmatch just ends up splitting up the team deathmatch players into two groups -- I really think they should have re-produced the Old School FFA matches. They provided an interesting alternative to regular FFA.
Among those modes new World at War are a Bootcamp mode for newbies, a Veteran mode for those level 32 and above, a Capture the Flag mode, a War mode that is Capture the Flag with momentum, and a Tactical mdoe that is for smaller groups.
Winner: World at War. Even without the old school FFA, there's enough new in World at War to give it the slight edge.
Weapon Selection. The basic types of weapons available are actually pretty similar with the ability to choose a sniper rifle, regular rifle, sub-machine gun, machine gun or shotgun. The weapons also have similar attachments, though the machine guns have tripods rather than improved grip. But, overall, I think the rifles in Modern Warfare bridge the gap between automatic and bolt-action better than in World at War, and the Modern Warfare weapons can be given a different skin to alter their appearance.
Winner: Modern Warfare. It has a slightly better variety and a bit more of a cool factor to the weapons.
Maps. This one is a pretty tough one because both games have pretty decent maps with nice little nooks and crannies that can be put to good use once you really learn the map. Also, personal preference and taste will go a long ways here. Overall, I think Modern Warfare does the best job of having well balanced maps where you can employ a variety of weapons and tactics on just about all the maps, while World at War has more variety even if some maps tend to favor certain type so of weapons a bit more than is found in Modern Warfare. It also does a good job at having expanded maps for team modes.
Winner: Tie. I'd favor Modern Warfare for strictly FFA mode, but when you add in the different team modes and the new maps World at War has that are specifically designed for those modes, it becomes too close to call.

Perks, Challenges and Kill Streaks. If you are hoping that World at War got rid of the Martyrdom perk, you are out of luck. Martyrdom remains in the game, and the perk system as a whole is very similar to Modern Warfare. There are a few new perks -- like the ability to put on some sunglasses to protect from signal flares -- but the ones most people are going to be using -- stopping power, juggernaut, double tap, overkill, deep impact, steady aim, second chance, dead silence, etc. -- they are all still there, and you'll end up using pretty much the same basic perks. The vehicle perks are new, of course, and you can now equip a flamethrower in your first perk slot, which can be fun but you won't get it until level 65.
The challenges in the game are pretty much the same, with most being used to unlock weapon attachments, and with very similar killer, humiliation and elite challenges to complete.
The biggest change is what you get for a 7 kill streak with dogs of war replacing the helicopter. (3 kills still gives you radar and 5 kills gives you an artillery strike similar to the airstrike.) The dogs of war are very cool -- when you set them off. Otherwise, they are mainly just annoying. Think about 10 times more annoying than the helicopter. Overall, they are bit overpowered compared to the helicopter. While you can at least kill them with your gun, you can't get away from them by simply ducking into a building. And the thrill you get from seeing all of those 5s fly across your screen might not make up for the annoyance of lining someone up in your sights only to have a dog bite you -- something that can happen quite frequently.
Winner: Modern Warfare, but only slightly.
Overall. Modern Warfare wins on 2 points compared to World at War wining just one point and both being tied for maps, but if we added Co-Opt as a point of contention, World at War would win it by default, so I am going to conclude by saying that they are both pretty evenly matched.
Modern Warfare still has an edge for those that like to go it alone. Not only are the Free for All maps in Modern Warfare pretty balanced and a lot of fun, but you can also choose between regular free for all and old school free for all. (World at War does allow hardcore free for all.) World at War gets the edge if you like playing alongside a team, with slightly more variety, maps designed for team warfare, and a co-opt mode.
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