I don't have a MDT in fact I am planning on getting an HDT. Keep in mind that any MDT or HDT for that matter is just as wide as the 550. The only difference is the 550 is a bit narrower in the front and the MDT/HDT are the same width from the front bumper to the back of the bed. You can also get the MDT with a shorter wheel base so its "foot print" is no bigger than the 550 and might even be smaller.
The MDT platform has a lot better visibility due to sitting higher, has heavier drive train components, and also has brakes that are more heavy duty, plus you can find an air ride MDT easier than an air ride 550 not to mention the power increase.
1996 Dodge CTD auto, #100 fuel plate,advanced timing,upgraded turbo, 2006 Dutchmen 28' 5th wheel with a 12' slide out and 16' boat behind it all