For my money, Halo is the better balanced of the two games, but then, this is based off my interpretation of what the word "balanced" means.
The problems with Call of Duty, were I to pick the most glaring ones with respect to this question, are fairly simple. First, the game world is fantastically lethal; any encounter between players is generally brief and lasts perhaps a few seconds at best. Because of this, the primary determining factor in the outcome of any gun battle is simply having one's weapon pointed very close to where an enemy will appear with victory most often going to the party who's weapon was closest to this unknown point at the outset. This means that the primary learned skill is simply understanding the common flow of players through a map. The result of this system is, quite simply, that an unskilled player is perfectly capable of killing a skilled player. In addition to this, Call of Duty includes a number of weapons capable of projecting enormous lethal effect across a wide area. This allows a player with little skill to simply shoot into a general area and kill another. The most troubling thing is that this weapon simply given at the outset of a life to any player that might want it. Finally, Call of Duty, as a game, minimizes a player's personal skill and contribution to a match yet, because of a number of factors, encourages a lone wolf style of play.
By contrast, in each of these areas we find that Reach offers some means of compensation. The dramatically increased durability of characters in Halo Reach dramatically reduces the chances of an unskilled player winning a battle with a skilled one by accident and the inclusion of a motion sensor goes a long way to making map knowledge a skill used for efficient navigation of a level and controlling certain spaces (and the weapons they contain) rather than one that very directly offers a tangible advantage to the outcome of any particular battle. Halo also features a number of area of effect weapons but these tend to be far pickier in their use. The rocket launcher is, of course, perfectly capable of killing a player with a hit in the vicinity but it is (generally) reserved as an ambient weapon and it's utility at longer range (which is relatively common) is suspect as there are more effective means of projecting hate and discontent. Of the grenades, only the plasma grenade is capable of killing a player in a single hit and even then only if the grenade physically impacts them. The game's direct equivalent to the noob tube is only capable of dealing instant death if you manage to hit a target directly (which isn't easy considering the relatively high rates of movement across uneven terrain inherent to the game). The game rewards personal skill and thus it's general encouragement of playing the game "lone wolf" is sensible and the game carefully enforces a team dynamic (if you want your team to succeed) in various modes through different means (by, for example, making a flag carrier all but incapable of defending themselves without dropping the flag and giving the enemy team numerous options of fast pursuit).
