My first piece of advise is don't do anything because "it is the last car" of the century.
My second piece of general advise is don't buy the first models of a new car line. Do as I say not as I do (my wife drives a late production 1998 ML320).
I have driven neither car so I can't make any comparisons. But after driving thousands of both makes and owning scores of MBs, I am currently driving a 97 540i. I have a new world of respect for the phrase "Ultimate Driving Machine".
I think the mid-size MB is the best car for almost every set of conditions. But I can tell you I sure like driving the mid-sized BMW.
BTW I am driving this car because it represents the greatest technical challenge I face in staying current with modern auto systems. BMWs use of electronics, computors, software, etc. is much more intrusive than MBs. Computors talking to computors and software/programming from the driver to the repair technician. I'm not sure this is a benefit, long term, to the vehicle owner.