As much as I hate the maintenance and reliability of both our X's , go with the 4.8 ... My wife's 3.0si is a great ride and I'm sure you'd be happy with it but there is just something special about driving my 4.8 (E53 though). Go out and test drive both you'll see the difference immediately.
If you get the 3.0 , don't test drive a 4.8 afterwards or you'll regret it
