Congrats on the new to you BX. There's been a lot of discussion on this point in the forums over the years. But let's see if I can add value in this one. I recently went from a RFM to a MMM on my BX and found out some things folks hadn't previously warned me about.
The MMM gets lots closer to the edges of things like fences, decks and trees. I have more control over where the MMM is than the RFM. But the RFM gets into corners better (I could back it in nicely). I'm at 7600' elevation. That means I lose a nice bit of HP on my 22HP baby. I was using a 6' RFM. I think I was pushing the power rating. The 54" MMM is better suited to my rig for that reason. I actually seem to cut quicker and cleaner with the MMM than the RFM. I'm better able to cut along the edge of the previous cut than I was with the RFM, but I'm sure that's more operator error/experience than anything else.
Now for the stuff that really surprised me. With the RFM most of the dust and debris is behind the tractor. With the MMM it's right under it. With the BX air flow being off of the operator deck it means you suck up lots more into the screens with the MMM than with the RFM. I used the RFM for 4 years with no problems with over heating or clogged screens. 2 hrs into my first cut with the MMM I found out about the difference /forums/images/graemlins/blush.gif Fortunately, an hour to cool down, some new antifreeze in the system and a good cleaning of the intake screens later all was well.
Price was a concern. New RFMs are slightly cheaper than new MMMs. The King Kutter XB48 will run you around $1500 while the Kubota 60" MMM is over $1800. You can find both used, but it's not easy. I was lucky. Another member of the forum in my area made me a deal I couldn't pass up on a 54" MMM. I'm loving it (thanks Ryan /forums/images/graemlins/smile.gif)
Good luck on your search.