What game should I buy for my new Xbox 360? Bio-shock or Mass Effect?

I own and love my PS3. I got the Xbox 360 as a wedding gift so i am looking for a good exclusive title to play on it. thanks!

if your looking for a scare type of game, go for bioshock. but if you want a longer game get mass effect

i've been playing bio shock for the past couple of days off and on. that is one spooky game. crazy people coming out of everywhere. the a.i. is good.
definitely a mature rating. you will jump a lot especially if you have the lights low.
have fun