MMM's are better for ease of use when cutting. On JD's (at least mid- to large- chassis machines anyway) you can use the mmm while the backhoe is attached. The JD hoes are 3PH and not sub-frame. RFM's are much easier to take off and clean, or so I have heard. It makes sense too. A MMM can be a pain in the tush to take on and off. It can also get really old after doing so many times if you need more clearance room or what not. RFM's are cheaper, too. I think the downside to an RFM is having to look behind you instead of just down when cutting. If you're cutting open areas, then that isn't a problem. Also, you can back an RFM in places that a MMM may not go.
Regarding the 4310, 4410, and 4510. If you are going to get hydro, the 4510 does not come with it. You will need to go with 4610 or 4710. If hydro isn't a concern, then there's no problem. lol The 4510 only comes with the PowerReverser. It is the next frame size up. Depending on the areas in which you will be mowing, the 4510 may be a bit big. 43, 44, & 4510 are all capable machines. It just depends on what you plan to do with it. There will be times where you wish you had a bigger machine, but I think that will happen no matter what tractor you get (any make or model).