Price-wise the Model X and the BMW X6 look comparable.

BMW X6 xDrive35i base MSRP = $59,600 and 0-60 in 6.3s
60 kWh Model X base MSRP = $59,900 and RWD 0-60 in 6.9s / AWD 0-60 in 5.9s

BMW X6 xDrive50i base MSRP = $70,200 :: 0-60 in 5.2s
85 kWh Model X base MSRP = $69,900 :: RWD 0-60 in 6.6s / AWD 0-60 in 5.6s

I am making the assumption that adding 15% (about 500 pounds) to the Model X will slow it down by about 1.0s in the 0-60 - that's just a guess.
I am also assuming if the addition of the second motor allows the Model X Performance to match the Model S Performance that the Model X AWD 60 kWh and AWD 85 kWh will match the Model S 60 kWh and 85 kWh 0-60 times - again just a (slightly) educated guess.

One big question is how much will the AWD option cost. If AWD's included with the base price then Model X dominates the X6. If AWD's a $10,000 option then you've got the 60 kWh Model X competing against the X6 xDrive50i which is a lot stiffer competition.

One thing I realize in doing this comparison however is that while the Model X and the X6 look very similar (perhaps they are the closest match to one another) the Model X is really a much different vehicle because of the Falcon Doors and the 3rd row of seats. I think a better comparison would be looking at the Model X versus luxury 3 row SUVs.

And besides the BMW X6 is only selling approximately 6,000 units per year so they are really more of a niche vehicle.