In my opinion, the truly great games in the future will have both. I prefer sandbox allowing the players to drive the community rather than being led around from quest giver to quest giver and isolated in instances. I like MMO's that feel like living worlds with constant change coming from what the players are doing in the game rather than everyone just waiting for new content from the Developers to drive thier enjoyment. I do think however the most successful games in the future will provide the flexibility to appeal players who enjoy to both aspects of MMO's.