Personally I would rather have the monthly fee. Every free mmo I've played I got bored with very quickly. Of course the game itself has to be good from the start, but without a game to base it off of, all I can do is go by past experiences.
I equate it to cable TV versus air. I'll pay for the clearer picture and extre channels. So what if it's a monthly fee, I get that much more enjoyment out of it.