No comparison, I drove my friends RRS supercharged and then got into my 4.8, the BMW handles much nicer, stops significantly better, and along the same road I could come off of one turn about 10 mph faster in the X5 than in the RR, the 4.6 isn't that much slower than the 4.8 and there is a huge difference btw the 4.8 and the RR, plus after driving the RR like I drive my BMW everyday the RR died, the electronics system fried and there was a complete vehicle failure (after 4000 miles), luckily my friend's dad owns the RR dealership so it wasn't an issue.

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2006 BMW X5 4.8is Lemans Blue/Beige/Dark Maple
-Nav, Comfort Seats, Heated Steering Wheel,
Rear Airbags, AUX Input
2009 BMW M3 Space Grey/Black/Carbon
-all options