Using Front and Rear Brake Pads with different hardness from different suppliers...
 

On the front disks of my 85 Mercedes 300D I used Mercedes soft pads but on the rear, I did them on a Sunday and actually used Pep Boys Pads, which are probably made out of Rock (ie as hard as...).
Recently while doing my wheel bearings, i noticed the three-month old Mercedes Pads were almost worn down!!
Then I realized the mismatch in Pad hardness might have resulted in harder than normal braking pressure on my Front pas--my rears had practically no wear--- Therefore should Mercedes Soft pads only be used when changing ou front and Rear.

PS How long should soft pads last?

http://mywebpages.comcast.net/rtc9/BENZ.html

In general front brakes account for 75% of the braking on most MB's.
The mileage that you get depends on driving style. My wife goes thru brake pads twice as fast as I do as she is always on the brake pedal.
Also as you can guess the compound of the pad does affect the wear.
As far as OE quality pads go I've found that Textar pads last longer. MB uses Textar on the rear of most newer Benzes