I mean no disrespect, but the MBCA discount of 20-30 % is peanuts compared with the savings you can find at any of the web based vendors who usually participate in this list. Not to mention FASTLANE! I usually save about 50% on major parts like water pumps, belt tensioners, motor mounts, etc...
Also, given the nitages of your cars, you're as likely to find a mechanic with experience on your specific model at an independent MB shop as you are at the dealer, again for much less.
I'm not trying to sell anything to anyone. I just saw a line extoling the economies of working with a dealer and had to throw my $.02 in to the mix.