I don't think it is that big of a deal btw the 4.6 and 4.8, only like 15hp, same suspension, the only difference is the adaptive headlights, the colors, and the rims appearance wise. It's really up to you.

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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