I have an '11 RRS Supercharged and used to own an '02 X5 3.0, so not a very precise comparison. I will say the maintenance is included on the new BMW and not on the RR.
I will add my previous RR was the full size '06 supercharged Westminster.
I absolutely love my latest RR and the reliability in this was has been flawless. Picked it out of the showroom floor in Oct '10 and they delivered to me. :-) We test drove the Porsche cayenne Turbo and the RR full size supercharged but not the X5 and actually enjoyed everything about this car. It handles well, has incredible power (510 hp) and feels very luxurious to drive.
I am a huge fan of the X5 but my gf is a RR fan and since this is her dd...
The electronics, nav etc is much nicer in the x cars. The interface for the entertainment in the RR is very lacking.
Test drive em both. Figure out what's important to you. I personally would be happy with either of them.