HST is a HydroStatic Transmission...basically a clutchless, gearless transmission (You still have to shift between High/Low ranges though). If you've ever used a garden or lawn tractor, you probably had one of these. It's somewhat equivalent to an automatic transmission in a car or truck, using fluid to transfer power rather then gears. The disadvantages are a higher cost (about $2000) when purchasing a new tractor, around 10% loss in usable HP (compared to a gear driven tractor) and higher repair costs if something goes bad (but there's a heck of a lot of old Deeres...755's, 855's etc, that have never required any kind of repair on the HST). Personaaly, I wouldn't spend the money on HST on a new tractor, but I wouldn't walk away from a used machine with HST. As far as resale value, the HST's may be a bit better, but basing an observation on two used Deere's I'm acquainted with there wasn't much difference as a percentage of the original price.
The dual-stage clutch allows the PTO driven implements to run when you shift gears. Pressing the clutch ½ down disengages the transmission so you can shift gears, but the PTO will continue running. Pushing the clutch all the way down disengages the transmission and the PTO. Pretty reliable system, but replacing that clutch costs around $1500. I've read life expectancy of a dual-stage clutch is around a 1000 hours, but that probably depends upon the competency of the operator. As with an automotive lutch, if one "rides" the clutch, it'll wear out faster. But an operator that knows how to use a clutch might get a lot more then a thousand hours out of one. For me, a thousand hours of use will be 8 to 10 years.
I use my 790 to maintain 2 acres. The other acre is all wooded. Frankly, it is a bit big for my needs...but I wanted that 24 HP at the PTO to drive a
chipper. Also, I do brush cut an acre or mine and about an acre and a half for a neighbor.
When I first looked into getting a tractor, my first choice was a Kubota
BX2200. For the majority of my work, that would have been fine...but it wouldn't handle the brush cutting I'm doing now. Also, for most of the loader work I do, the BX wouldn't have been enough. I must admit, when the tractor gets bigger, the projects do too.
I really enjoy my 790. Some of that enjoyment is the kid in me... It's a good tractor for me...sounds like you're in similar circumstances, so it ought to be a good tractor for you.
The way I read your post, it seems your neighbor must be in agriculture since he has more then one tractor. Well, the 790 isn't so good at that...too small. The guy who had my tractor originally sold it to buy a bigger machine (990).
But, for a residential or small (hobby) farm user of no more then 10 acres, the 790 is a great tractor. If the machine you're looking at doesn't have a mid-PTO, that can be added for $400. You can add several front attachments including a snow blower ($$$ though).
Good luck with your decision!