I really wouldn't have been in a quandry one year ago. Our ML320 has a pretty plain interior with too much plastic. The Gelandewagon, on the other hand, has an interior in the "classic" Mercedes-Benz style (like our E500). I was saving for the G-wagon.
Now M-B seems to have figured out the "perceived quality" problems of the ML and, from the magazine pics I saw, the ML500 is as well put together inside as outside. Combine its lighter weight, same engine, modern styling...now I have a problem choosing.