The V8 is a beautiful machine, but if maintenance cost is a large part of your decision making, I would strongly suggest the I6. Solid proven engine (won best engine in its category many many times). I have an 03 3.0 and while I have had a fair share of issues, they have all been small and relatively inexpensive. More space to work in as well. Good luck, I wouldn't change my X5 for anything; bugs and all.

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'03 Black BMW E53 3.0i 110,000+ miles
'03 Black Mercedes-Benz W211 125,000+ miles
'13 White Mercedes-Benz W2o4 New