I have used my Gravely 1740 for three years with no problems. Well almost, if it sits for more than two weeks the Kohler Courage engine can be hard to start. I think this is gas draining out of the lines back into the tanks because if I put the tank selector on off it will quickly fire up the next time. Now if I could just remember to do that each time I put it up.
As I have stated in other post my yard is 1.1 acres with (currently) 21 trees, 2 stumps (waiting to be removed), 2 flower beds, 2 fish ponds, 2 patios, and 3 buildings counting the house. Also paved drive, gravel drive, retaining wall, sidewalks front and back, and three board wooden fences on sides and backs. In other words a lot of crap to mow around. My previous mower was a 17-hp lawn tractor with 42" deck. It took from 1:45 to 2:30 to mow depending how close your got to trees and such. Now I can mow the yard in less than an hour depending upon the pattern or stripes I want to leave in the grass.
As far as quality of cut a new Murray with a good unbent deck and sharp blades will cut as well as anything if driven slow enough. The little Gravely will do a good job.
Disadvantages of a ZTR are several. First they do not like wet, at least mine does not. There is on spot in particular in my yard that stays wet due to field tiles. I have to go full speed and hit it straight across and the little ZTR will still slide sideways until momentum carries me across. Also beware of slopes when wet. Had to pull it out of a pond at my father-in-laws house because of this. I had mowed that small hillside half a dozen times with no problem until one day it came a small shower and I found myself sliding sideways about fifteen feet until the mud stopped me.
From the description of your lot I would not get a lawn tractor. You will spend more time backing and turning than mowing. Go with a small ZTR or a larger walkbehind mower. From my experience with the little Gravely I would reccomend it and I understand they have a special promotion running until the middle of this month.