He has done both but is better off the striker. Laudrup seems to agree:
"Michu is an offensive midfielder who can play centre-forward but he will be a goalscorer wherever he plays.
"He is not only a goalscorer who is up there, waiting. When he plays in midfield he is working very hard and physically he has a presence and fills a lot of space as well.
"I see him more as an offensive midfielder. He can come in from that second line where he is so gifted."
http://www.football365.com/news/21554/8872389/Michu-Not-For-Sale