Agreed - Metallica over both at this point.
Both are fun games. If you like a challenge then IM would be the better pick as it can just be brutal. If you like a bit more depth then ST is the game. Right now the code in ST isn't all that great. It's decent but not as polished as it should and will be. I think ST has the greater potential but comparing the games as they sit today I would go for IM.