Well, if you have money burning a hole in your pocket, by all means I dont want to stand in the way of you and a new cutter.
But from a logic standpoint, it dont make much sense. Its like trading in you tried and true 10 year old wood hauler for BRAND NEW truck to go to the woods and beat to heck hauling wood. Just makes no sense.
The MD cutter you have now, how much are they giving you in trade?? Cause I am betting it isnt much more than $500 based on this
http://www.tractorhouse.com/listingsdetail/detail.aspx?OHID=7100681&dlr=1&pcid=3074872
And I would also bet that in another 10 years, if you take care of the cutter, it will still be worth that $500.
Heck, in my area, I see 30 year old cutters without a trace of paint left and beat up pretty bad go for $250. So I dont think the value is going to go that much lower like you think.
But again, if you just ahve to have that new cutter, by all means. They are both good choices, but given the price, I'd opt for the bushhog. But then again, with your tractor, I'd be looking at a 6' cutter also.