I have a 2001 4.4i and two other BMW's with the inline six. Overall for the X5 the 4.4 V8 seems like the way to go for overall satisfaction. But I can say that the V8 does require more maintenance than the six's.

I just had the intake gaskets, valley pan gasket and valve cover gaskets done on mine, but I had done as JCL pointed out and gotten an extended warranty which covered it all minus the $100 deductible. This makes the 4.4 an easy choice in my opinion.

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2013 X5 xDrive3.5i Premium
2002 530I Sport Package
2001 X5 4.4i
2001 M3