both.
What I'd enjoy most is a story driven MMO for leveling and even adventure later on, plus the possibility to go off and do my own thing, when the story ends or i'm bored with it.
sandbox without story is too much like real life, i already have to compete there, when i play a game i want to get away from that.
Themepark with only PVP as endgame is boring as well, i don't like PVP so if i cant do anything else i stop playing and paying.
I want to explore and be entertained.
It's a shame that so many games offer thousands of different ways to kill something, but almost none to explore, find something beautiful, mysterious or funny or create something nice or meaningful for the game-world.