Utility
The BMW X5 will most definitely prove handier than the BMW X6 for those of you with frequent child-cargo.
The BMW X5 is a better choice than the X6 as far as transporting cargo, especially if you often need to carry a lot of luggage or big pets.
Performance
With respect to horsepower, the X6 packs quite a punch and will knock the BMW X5 down a few rungs. However, remember that high horsepower engines often command higher insurance premiums.
Dimensions
The BMW X5 and the BMW X6 compete for the same parking spaces.
Convenience
You'll most certainly have to take the BMW X6 for a fill-up more often than the X5 because of its tank size and fuel economy.
Cost
The X6 uses a lot of gasoline compared to the BMW X5.
With respect to fuel economy, the BMW X5 beats the BMW X6.
The BMW X5 is substantially less pricey than the BMW X6 in terms of MSRP.
The BMW X6 will cost you a lots_of_money in gas every year compared to the BMW X5.
Drivetrain
The BMW X5's engine isn't much larger than the BMW X6's.
The BMW X5 could learn a lesson or two from the X6 with respect to pickup.
The X6 packs quite a wallop compared to the BMW X5 when it comes to torque, which is roughly equivalent to acceleration.
Comfort
Taller drivers will substantially enjoy the extra head room in the BMW X5's front cabin compared to the BMW X6.
The ampler interior dimensions of the BMW X5 make it considerably roomier for your passengers than the BMW X6.
Warranty
The BMW X5 and the X6 have the same basic warranty.