I was talking mowers with a friend of mine the other week. I was using a SD the dealer lent me until my RD RFM came in and said to my buddy that the only thing I didn't like about it was that it seems to leave the grass in clumps. He used to own a landscaping business and now owns a campground (lots of acreage to mow) and said you won't see any landscapers using a RD because the SDs cut the grass finer because of the mower deck design and the blades throwing the grass towards the chute. I think my tractor tends to lug down more with the RD, too. There's lots of pros and cons for both. There was lots I didn't like about the SD while I was using one.
As far as mid or rear mount, the only advantage I can see to having a MM is you can attach a roller and roll your grass in the spring while you mow. The RM on the other hand is great for getting under bushes.
Fred