For open field bush-hogging, the gear has an advantage due to greater efficiency, but in the woods and cutting around trees, etc, the hydro would be just as, if not more efficient. I would let that decide for you, if you are doing mostly open field work - go gear, otherwise hydro. I have a 4120 gear and it has power to spare on a 6 ft bush hog, even cutting thick stuff so I think, even with the slight efficiency loss of the hydro, you would still be ok with a 6 ft mower. I use the 4120 mostly for open field bush-hogging and some tillage (plow/disc), so the gear decision was a no-brainer for me. That said, I have both gear and hydro lawn mid-mount mower tractors of equal hp, and I can get the lawn cut faster with the hydro even though it has a narrower cut. I have a lot of trees, buildings, etc to mow around and the hydro is a huge advantage in that situation. If you think you will get a cab, and hydro, a 4320 might be in order as the AC will suck up a little more hp and ,coupled with the hydro efficiency loss, you may be pushing the power a bit for a 6 ft bush-hog. The 4000 series are a little too wide for a 5-foot mower. Personally, I feel a canopy is a much better proposition than a cab on these little machines anyhow, if working under the hot sun bothers you. A good canopy takes care of that problem, dont cost much money, and is easy to take off in a hurry if you need to fold the roll-bar for clearance issues. If you go for a 4120, I would also recommend the R1 tires with the rears loaded. That will give you enough traction to get all the tractor's hp into the ground if you ever need it. R4's will really limit your drawbar pull ability on soft ground. There really isn't much difference in damage to a soft lawn between R4's and R1's, but there is a huge difference in traction ability on soft ground.
