Both engines have their sets of issues. But as a general rule, V8 will cost more than the TT I6 to maintain. Both can be expensive. If you go in budgeting $3-4k/yr in maintenance costs for the first couple of years you'll be fine. After those first couple of years, they tend to get easier on the wallet as most of the normal issues will have been addressed. If you expect to drive them worry free with just oil changes and tire rotations, you've picked the wrong platform. The N62 V8 torque is addictive.

TLDR:
V8 = more fun and more $$$
I6 = more reliable (in general) and "cheaper" to maintain

Whatever you buy, get a GOOD PPI. Even if it means spending $200 and then walking away from the vehicle.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)