I have also been thinking about this for a while.
I have decided that the AMG style mufflers, may or may not sound very good, you would have to buy one and install it to really see.
I have decided I would rather go to a shop that builds custom exhaust systems for all kinds of cars, that has all the necessary pipe bending equipment and such, and have them put in an exhaust based on how I want it to sound.
My brother just had a magnaflow muffler put on his range rover with dual tips, and the thing sounds great, but then again, it sounds liek ti should have sounded fromt he factory, Deep and mean, just like a V8 should sound.
I saw a Brabus CL500 today, with the dual exhaust tips on both sides, the thing sounded amazing.
I also saw a brabus C280, full brabus, right down to the body kit, wheels, exhaust, and badges, and the car sounded sweet. The guy took off and was gone so I am pretty sure the engine was modded too, but the car sounded nice and throaty.
When I do my exhaust on my benz, I want it to be throaty upon heavy accelleration and not too loud anytime else. I think the only thing to do is to go to a pro to be able to describe how I want it to sound.
Have you thought about removing the center muffler or resonator on your car and replacing it with a Y pipe?
I think that will give you a little more sound, without being majorly overdone.
Otherwise go with a brand name exhaust, I think those clone exhausts, probably don't last as long or work as well, but they do look great.
I saw some nice universal mufflers in the latest JC whitney sports compact catalogue, and found a nice looking one which I like the tips better than the other one I had previously liked.
I don't think you can go wrong with spending a little extra dough on a much higher quality muffler, but like I said I have not heard or seen the amg style mufflers to be able to tell you what they sound like, but I do know the real thing sounds sweet.
Alon