At the beginning of last season I said behind the striker, and the beginning of the season vindicated me for a bit.
But having seen him play both positions a considerable amount, I'm leaning towards deeper now. The cons are that he occasionally will lose his runner leaving us exposed in the back, that his height isn't as much of a factor as it is when he plays up top, and that he is more likely to draw cards in that part of the field.
The pros however, are that our entire style of play is more positive and i believe more effective when we don't have him as a target up top, that it frees up our attacking midfield players like osman, mirallas, and pienaar to work those sexy slick passes, that we'll win almost every header in midfield, and that with gibson sitting he can still occasionally make runs into the box and will be harder to track when he does so.
Hopefully martinez makes use of his versatility properly. Moyes would change him around too much, and you could tell that it threw him off occasionally. Fellaini's primary position should be holding midfielder, but in a game where we concede early but enjoy plenty of possession, he should push forward.