I own both games. Both games are incredible.
Mario Galaxy gets my vote because it's the one I got the more enjoyment from.
When I finished MGS4, I started a new game right away to play around with the Psycho Mantis doll but that lasted only a couple of minutes.
When I finished Mario Galaxy with 120 stars and immediately started a new game with Luigi, I stopped playing when I was at about 60 stars with him.
I would also like to say that neither game is the best of their series: the best MGS is still the first one and the best Marios are SMB3, SMW and SM64.
Signature goes here!








