I had a 3320 with the 72" 7 Iron MMM. Moved the mower to a 3720 cab and it was a pain so I went with an 84" RFM. (I'm now upgrading to a 4720 cab but that's another issue

).
MMM pros:
Great cut quality.
good maneuverability 'around' things
Could leave rear implement on.
You can use it with a material collection system like a cyclone rake or something.
Easy to access/maintain but only when off the tractor.
MMM cons:
A pain to take on/off, especially with wheel extenders.
You don't want to do much serious 'tractor stuff' with your $3000 deck on the tractor
it can get banged up easily. You will have s#$t for ground clearance.
Compared to a RFM, the wheel stance is narrower and will 'rock' more as it floats on the
tractor.
On the 3000 series you have to lose your height adjustment to take the deck on/off.
Also, it's very sensitive to leveling - I found I needed to level it each time I mounted
it.
You aren't going to mow 'under' branches unless you can fold your ROPS real low. If you have
a cab you are SOL.
RFM Pros:
You can mow under branches by backing in.

Easy on/Easy off - No problem doing tractor chores.
Good cut quality (Mine is almost 18K feet per minute tip speed, faster than my MMM)
Easy maintenance
Pro/con - Wider wheel stance = more stable, less rocking but also more prone to
scalping. On the other end, the front/rear wheelbase is shorter so that's less prone
to scalping than a MMM.
RFM Cons:
If you have a lot of obstacles requiring frequent short turns you'll be unhappy.
if you get a rear discharge you cannot blow the clippings and re-cut. However
the mulch kits work pretty well (I have one on my 84" RFM and one on my 62C on
my 2305). You can also get a side discharge RFM but they usually don't go larger than
72"
Doesn't hide under the tractor so you'll need space to store it.
Don't turn sharp when up against your car or anything else...
