I drove both back to back before I bought my X5. Although the 4.4 is great, the 4.8 was that much better. The 4.8 just felt like it could go and go. I am completely satisfied. Shop around, though--and get the best deal that you can.

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2005 X5 4.8is/ Imola Red/ Cream
2002 ///M Roadster/ Alpine White/ Imola Red (SOLD!)
2008 ///M5 ordered and on it's way!!