In my experience, classless=mage wins every time; however, that isn't necessarily always true.
I've always preferred classless for the customization, since many of the workarounds for multiclassing in class-based systems don't really tickle my fancy. I like to have my barbarian know a spell or two or my wizard also know how to fight with a polearm, that kind of thing. I also like how much less thought goes into creating a character in a classless system since you always get all the options, there are often fewer options. In a class-based system, you have to look at all the classes and see which you like best, whereas in a classless system it's easier to say "I want to be this kind of mage. Done." In a class-based system it's more "I want to be this kind of mage. Well, this one is sort of like it, but so's this one. If I mix and match I can kind of have what I want."
I guess my argument is saying the opposite of what Smoss said. I've always had to take longer on class-based systems than on classless systems to make my characters; though, I do agree with Smoss that player choice paralyzation can be a problem. I think it's a problem more to do with rules-heavy v. rules-light systems rather than classless v. class-based; however, my experience has shown me that classless systems usually have fewer rules, which is probably why I take less time with them.