It really depends on what your looking for.

I recently bought a 3.0 X5 this was purely because of fuel consumption but also insurance as I'm only 26. However given a free choice it would most definitely be the 4.8is

Think about what is most important to you. The 3.0 is sufficient for me, but it does lack the oomph you sometimes want, but having said that I'm extremely grateful I have an opportunity to own an X5 at 26!