I've always wondered the true Pros/cons from these three. Try to put as many things as you can.
(Aargh my English >_<)

Free:

+Simple
+Don't have to write so much(I'm going to get a lot of hate from this)

-Can sometimes go too fast
-Many rps die
-High skill difference between the players

Advanced:

+High character development
+Good storylines
+Turns are controlled(?)

-Umm... You need to write a lot

Note: I only have played free thus far, so I don't know everything about Advanced and Casual