That is a good question, especially on a tractor the size of the BX. One guy I know uses a RFM on his BX for mowing around his Christmas trees. Grass clippings, blown on the lower branches will kill them, so the rear discharge is a huge advantage for him.
If you have a lot of small rocks, trees, etc., to mow around perhaps the MMM is better. However, if you have lots of bushes, shrubs, or trees with low branches, it is much easier to sneak a RFM under there.
Another consideration not often talked about is traction, and stability. RFM's ride on the ground, so you are pulling that weight, and on side hills, that weight is free to swing down hill. I would think that front weights would be a good idea since RFM's tend to be heavier than MMM's, and surprisingly, sometimes more expensive. The MMM on that tractor is a suspended deck, so the weight of the mower is added to the weight of the tractor for stability, and traction. That weight is also below the axles so it lowers the center of gravity.
MMM's are specific to the model tractor you have, and can't even be interchanged between the early BX models (
BX2200, etc) and the current 50 series tractors, nor between the
BX1850, and the
BX2350. A RFM can be used on all tractors that share the hitch size it was designed for.
Woods, and Landpride have very nice RFM's designed for your tractor, and they should last every bit as long as a MMM would. The only one I've seen on a BX is the Woods rear discharge unit that the Christmas tree guy has, and he has been very happy with it.
Good luck with your choices!