I like competetive more, but only as part of a semi-functioning team. I like support roles, and rarely take the initiative to charge ahead by myself, because that usually just gets me killed without any noticeable benefit. I like to think on my feet, and not just rush ahead without any idea of what I'm really doing.
Co-op is just "okay" for me, because most of the time the people I'm trying to help are getting themselves killed, because they can't seem think on their feet. Or they like to prove how "tough" they are by charging ahead of everyone else, and soloing it to the next exit. L4D is great with friends, but like Kodolb. said: all it takes is one idiot to ruin it for everyone else. Army of Two had some fun MP moments, but I also remember having to pick my "buddy" up more often then I can recall, because he / she didn't have any basic survival instincts. Come to think of it: I had to do that quite a lot in L4D too, which is probably why I haven't played that game in a while...