It seems that the main use for this tractor will be mowing, and the main use outside of that is for trail work in the woods, and perhaps some firewood cutting, etc. The amount of acreage has little to do with tractor size in this case, and though 21 acres sounds like a lot to many, much of it might not be really accessable. My woods are full of places I don't care to take any tractor, and boils down to having something large enough to drive over "stuff" ( rocks, bogs, tree stumps, etc) or small enough to drive around it. I know logging skidders have been in there a couple of times, and I have been there with my BX.... didn't work so well because of clearance issues, and my
B2620. I've used my 4' box blade in there (might have been able to use a 5'), my 4' rotary cutter (don't want anything larger than that!) my forks (work just great, but have to be raised, and curled back to turn around in places), and my
chipper ( with forks, or loader bucket make the unit as long as I care to have it, and I have to choose my turn around spots carefully) In my yard which is over three acres of obstacles I wouldn't want anything larger than my B except for backhoe projects. The B is very nimble, but is rough on the turf when you try to use all the nimbleness that it has. The outside of the front tires does a number on the turf in tight turns. I think the JD's are better in this regard though.
I take my mmm off my garden tractor every 25-50 hours depending mostly on how well I'm feeling. It is only a 48" deck but it is commercial construction, and is heavy with a full width rear roller. That is about as often as I really care to be messing with it. I use that tractor for mowing, collecting grass, and leaves, aerating, dethatching, and pulling a trailer, especially when I'm chipping brush, and fallen branches around the yard. For me having that second small garden tractor is almost a necessity, and so a Z-Turn wouldn't work. Having both is perfect! Had I known that I probably woudn't mow with my "tractor" I might have bought one size larger (
B3200,
B2630, JD 2520,JD2720), especially if I decided to buy a backhoe (which I miss sometimes, and is usefull in trail building / improving work, but I sure don't need them for the larger attachments. If I were to move up a size now, I would just keep the implements I have. It's true that with something large enough, one can just overpower many tasks, and get them done now. I have an engineer friend that has 50 acres, or so of woods, and he likes much larger tractors. For me, my 13 acres is a hobby, and playground, and I don't mind a few more trips back, and forth to the gravel pile when I'm puttering in my woods. The difference is that my engineer friend needs a "road" to move his industrial backhoe around, and I enjoy my trails. Frame of mind, you see, has more to do with it than anything else.
So, choices? Well for mowing: easy to maintain, heavy enough to be stable, and rugged (850-1100 lbs perhaps) easy to attach, and efficient material collection) snowblowing option, and comfortable to operate. Ideal......... JD X700 series, or possibly Simplicity Legacy XL. Acceptable........ JD X500 series, Simplicity Prestige, or equivalant Cub Cadet, etc.
Tractor duties: Simple, tough underbelly, and comfortable to use. Tossup on brands here since the JD's have better steering brakes than Kubotas, and Kubotas have 3 speed hydros, and tougher underbellies. I like the Masseys, Mahindras, and to some extent the Kioti / BobCats too, but most come with less options, are not as nimble, and around here are the same price, or more than the JD, or Kubota with lower resale values. In larger sizes they have much merit, and better price points.