I have both; MMM for the BX and kept my old Wheel Horse garden tractor, which is not really a cheap mower but is a little rough by today's standards. I almost always mow with the WH since it's all set up and it's just easier to hop on and mow with it than convert the BX. In my mind, BH and FEL removal are not too bad, but hooking up the MMM is more work. So, if you think you can keep the MMM on the BX while doing loader and/or BH work, it might not be too bad to drop them and go mow with it, but if you have to remove the BH and FEL, then mount the MMM to mow, then reverse for other chores, it will get old fast IMHO.
I would love to mow with the BX as it is an awesome mower, but my WH ain't too bad either. Lately my issue is spending the money to gas up the WH since it goes through gas a lot faster than my BX goes through diesel, but a newer mower should be more economical than my old 20 hp Onan. I figure in a few years when most of the FEL/BH work is done, I'll be able to keep the MMM on more and use it. It's also nice to have a backup machine, especially when one is 18 years old and has the occasional mechanical/electrical issue.
Another consideration is if you have room for both, but I assume you do if your considering a separate, dedicated mower. Also, you can pick up some nice, quality used mowers for less than $2K as another alternative. (Just some added thoughts.)