What game should i buy halo 3 or rockband?

i just bought halo 3 today but i havent opened it cause i havent made my mind, either to keep it or to return to buy the game rockband, but if i return it aand buy rockband then i have waist another 60 dollars to buy the guitar. can someone tell me what game is better and if i should waist 60 dollars on the guitar?