Interesting thread this, particularly with respect to fuel consumption of the engine variants for the e53 series. I know my 3.0i needs more acceleration to get going because the x5 body and it's multitudinous parts is heavy and requires power from the engine to get it going, whereas the v8 would do this with less effort and save on fuel. Fuel consumption is a big factor when purchasing a car! What you save in petrol can be in the 1000s per annum, and what you save can go towards paying for cost of service.
It is down to practicalities. If you need your car to haul the family around on weekends and there is a big dual wheel caravan that you take on country holidays, and this car is also needed for quick overtaking of other vehicles on the highways to get to work on time, then definitely v8 is the go definitely, as you will find the 3.0 a bit slow off the mark, and you will be burning the fuel with all the stopping and starting @ traffic lights.
If, however, your use of the car is less demanding, such as weekend use essentially with a small family such as mine (1 child) and the occasional country trip with a small caravan, then the 3.0 variants should save you on petrol in the long run, and believe you me, beyond 120 km/h, the 3.0i just hums and burns less than 10 ltrs per 100km.
It is a known fact that all BMW cars "sing" and feel very comfortable at high speeds because they hug the road with their racing class stability and there is no vibration. It is part of the reason why BMW cars are expensive to buy and service, as it is all those wonderful parts working in unison to create the magic.