I owned an X6 for a few years. In retrospect I should have gotten an X5.
The X6 costs more, has less room, drives about the same, same MPG, no diesel option in U.S., and has a very cramped back seat.
I would stick with an X5.
2015 435i Convertible
M Sport, M Performance Power Kit
2016 Lexus CT200H
2016 Lexus NX200t