There are so many MU servers out there its not funny.
From what I've seen, there are not as many PT servers. Its a fun game is the XP is multiplied enough, because if anyone hasn't played it, it takes a lot of time to level the higher you get. More so then most games I've played.
I played KO once, got to level 10 and forgot about it, so I don't exactly remember the pros and cons for it.
I voted for PT since MU is vastly popular and don't know much about KO.