To better understand the diesel advantage look at the only diesel offering by BMW in the US at the moment (a 328d diesel version with 184 hp will be offered) and just compare it to its petrol counterpart. Despite the huge weight disadvantage of the diesel version (although with a better weight distribution), the lower power output and the 6-speed only transmission when compared to the 8-speed automatic transmission of the petrol version the performance of both versions is remarkably close!
What set them definitely apart is the mileage you can get with each of them. And, if you take both versions for a test drive on the same route and drive them hard like a 'true sports car' I can assure you that with the diesel version you will get the DOUBLE of the mileage you will get from its petrol counterpart. GUARANTEED!!! 
.................................................. ............ X5 xDrive35i........................ X5 xDrive35d (diesel)
Weight distribution, front/rear................. 47.3/52.7 %......................... 49.0/51.0 %
Weight Unladen........................................ 4960 lbs............................... 5192 lbs
Nominal output hp/rpm........................... 300/5800-6250................... 265/4200
Max. torque lb-ft/rpm.............................. 300/1300-5000................... 425/1750-2250
Top speed (limited electronically)........... 130 mph.............................. 130 mph
Acceleration 0-60 mph............................. 6.4 sec................................. 6.9 sec
Fuel Consumption City/Highway............. 16/23 mpg........................... 19/26 mpg