I use a rear mower. Never used a mid-mower, but since the mid-mower is dedicated to a specific tractor or model of tractor, I doubt if I ever do (unless I buy a machine the already has one installed).
With a rear mower, you can back down slopes or under branches. For me, this is a significant advantage.
However, although I can get right next to a fence or the house, I probably do a bit more trim work then a person using a mid mower would have to do.
As far as watching the mower, once I make those passes closest to the house, outbuilding and fences, I rarely have to look back (unless I'm backing up, of course).
Rear mowers (Finishing or Grooming Mower is the proper terminology) are quite a bit cheaper then mid-mount mowers, by the way...
And, Henro makes a good point...get as wide a mower as you can (and your tractor can lift). I'm using a 60" mower (owned before i got my 790). I could use a 72" model with no problems...but I don't expect to replace my current mower this year.