I think paid MMO are generally better because more levels are added to the games on a regular basis. But, free MMO are good if you don't have much money, and don't mind grinding more, so you can level up without buying in-game money or items.
I tend to like paid more also as they tend to have less grinding. It seems like free MMO's make it take longer to achieve things to tempt you into spending money.
I've never played a paid MMO in my life, so I can't really compare them to free ones. Seeing how my financial situation is pretty terrible, I tend to spend as little as possible online...