First point of discussion are the virtual worlds you reside in. Now in sheer size It is difficult to tell because I have only explored so much in any of the two. And it seems that the maps are smaller in WoW, ofcourse dungeons and cities are huge, but in CoH the dungeons can be quite big as well. The maps in WoW might seem large but they are mostly not that big, once you get into a new area it is quickly explored and your whole map is discovered. While in CoH there is only one city divided by districts, which are really huge and when you have no transportation powers at the start of the game it is frustrating because it takes so long to get somewhere. Getting the whole map explored is also not as easy as it sounds, some places are even so hard to discover you get awards (badges) to do so. The quality of the virtual world graphics is best in WoW, the environment and dungeons are rather dull in CoH, but at the same time it never stands in your way or is really ugly, lets call it simple but functional. This is valid in both worlds, except I found the player characters in CoH to be much better then WoW. There is almost an infinite number of possibilities to change the look, which is really nice.
This brings us to the character development, which is one of the important parts of playing RPGs. I received CoH one month in advance because some delays at amazon, where I had ordered both games. So at the beginning I had only one game to play, and got a nice head start with CoH. I always though my character would me more developed and higher level in CoH, which turned out not to be true. Character development in WoW is fast, XP is received constantly and when you are 'rested' it even speeds up the whole leveling process. This is good, because at each lvl-up you get something new to play with, new powers, enhancements or possibilities to wear better gear etc. As everybody knows by now, WoW doesn't know debt while all other MMORPGs use this particular system. I'm not sure yet if I like this or not, a lot of times you will get yourself into a dangerous situation and if you don't make it, you don't lose anything (except 10% of equipment endurance). So where is the danger? The worst that can happen is some downtime/delay before you can resurrect again. While in CoH people are still punished for getting them self in a bad situation and must plan their attacks better, play more as a team (or at least try) or fear to build up debt upon debt upon debt.
As versatile as the look of the characters is in CoH, at least as different are the possibilities for adding super-powers. The creators clearly had in mind that each player should be different and although some powers are very popular, it is again almost impossible to have a hero with exact the same powers, just like in the comic books. In WoW you can differ your character mostly by specializing in certain talents and the equipment you carry, but this does not even begin to compare the diversity of possibilities in CoH.
Anybody playing CoH will agree that in the lower levels endurance is a real problem. During fights you will always run out of endurance, which each class uses to perform special powers, it is the same thing as mana in fact. This problem can exist in WoW as well, except they have developed a genuine system for fighting classes like the warrior where you have to build up rage to perform your attacks. Thus preventing, that type of class at least, to run out of mana. I feel like this should be incorporated into each future MMORPG, as it makes more and more sense after playing like that for a while.
Missions are done differently in CoH then WoW, that is my impression at least. WoW will allow you to have up to 20 open missions at a time, you can in fact mix story lines and do each mission in the order you like the most or is most convenient at that time. While playing in CoH I tend to stick to one contact providing me mission after mission and following the whole storyline in one go, certainly gets you more into the game at some points.
When WoW was released, it was marketed as the 'quick' MMORPG where you could jump in for an hour and do something. I don't see it that way, the only straight-on game is CoH. It is made simple, there are no extras but contains enough to play it for short sessions or hours on end. WoW is really a time sucker if you want to get something done, and there is too much to do and maintain or take care off. The only thing that is correct is that levels advance pretty quick, at least in the beginning they do.
Both games get updates at regular times. CoH has already doubled in size compared to the first version, and we all know blizzard won't sit still adding new features and other things to WoW, just like they do with all their other games.
So which is my favorite MMORPG? Hard to say, they are both very good games. And perhaps my preference still goes out to CoH. It seems the people who play that game are more friendly and willing to help each other (just like real heroes), you can be sure you will be adding a lot of people to your friends list in CoH. I never had that in WoW, there are parties but they contain no spark. It is fun playing WoW because the game itself is really well made and a lot of people are playing that game, and in fact all my real friends play it too.
