Think of the X5/X6 relationship as being the same as that between the 5 and 6-series. The 6 is positioned as the sportier, slightly more upscale model. Auto Zeitung is predicting the X6 will share the best engines from BMW's range including the 286hp twin-turbo 3.0 liter diesel, the 335i's 300hp twin-turbo 6, and the 4.8 liter V8 putting out 367hp. Word has it a turbo V8 with 420hp is also in the works and should be good for a sub-six second 0-100 km/h sprint.
Like the X5 4.8is, BMW will most likely stay away from the "M" label for that model. Whichever engine you pick you'll get the power routed through a 6-speed automatic with that new joystick shifter from the 2007 X5 that's promised for the 5-series as well. Looks like crossovers like this are the future for many automakers, as Audi will have the Q5 and Mercedes the GLK in the next couple of years, too.
[Source: Auto Zeitung via MotorAuthority]