I was in the same boat as you back in Nov. wether to keep 98 ML320 with 37K mi. which was paid for, or sell/ trade it in. I was looking at the 02 ML500 because ML 320 engine not enough. Great gas mileage but that's not critical. Wanted comand system and was deciding between ML 55 leftover 01.
So now let me say that everyone that posts a comparison of an AMG modified model to another is comparing two totally different cars. It's like trying to figure out wether to buy a sports car or a sedan.
The ML55 will not give you good gas mileage for your long trips, the seats are different and better and of course the ride. My wife always complained the 320 seats in the back were hard and you could feel the bumps hard. When we test drove the 55 she loved it.
Even the ML500 is a different car than the 55 because it is very good better value than the ML430. Make sure you get Comand if not all the other goodies.
01 ML 55 700 mi.
98 ML320 40K
87 560 SL 17K- cond.1