I havent played either lf the two games, but im gonna have to vote for Diablo 3 because I enjoyed Diablo I and I've never played a Mario game
My prediction threads:
Wii U will sell under 40m units (made on 14th September 2012)
PS Vita will sell under 20m units (made on 30th September 2012)
Wii U will sell under 7m in 2013 - I was right