I have the 3.0 and the 4.8 X5's and the 3.0 has plenty of power. The 4.8 is a more effortless power. Just touch the gas and gone! Unless you really want the extra power, the 3.0 is fine. The 3.0 feels easier to drive for some reason too. Weight? Maybe I'm just used to driving it because it's older and been around longer and just feels more familiar, I don't know. I'm sure either one will be ok as long as it's got the options you want.

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2007 X5 4.8i Space Gray
2001 X5 3.0 Titanium Silver