The 790 uses a dual stage clutch (½ down stops the motion of the tractor, all the way depressed stops both tractor motion and the PTO).
However, that's an option with the 2WD 790 (which is what you've priced, I believe). Otherwise, you gat a standard "non-live" PTO.
I've read about the Kubota 2800. Powerwise, it should be pretty close to the 790. I was of the impression one could get a dual stage clutch on these machines too, but I could be mistaken.
As far as using a single stage clutch...well, it does stop the PTO when depressed. That's a pretty easy thing to adapt to though. I had a single stage clutch on my 670. Although I do prefer the dual stage (until it comes time to replace it!), using a single stage wasn't bad. You might want to plan your mowing pattern to reduce stopping or having to clutch. It may require backing up now and then to hit the spots you missed when clutching. This is for finish mowing.
For brush cutting, the inertia of the blades kept them moving pretty quickly during any gear shifting. Heck, it takes a good 60 to 90 seconds for those blades to stop rotating. If you can't shift in that time...maybe you should think about a hydro tranny!
Might want to price what the optional dual stage clutch will add to the 790's base price. If it's more then a grand, you might want to go a bit further and go for the MFWD 790 (dual stage clutch is standard on that machine).
You might want to find out if the dual stage clutch is availible on the Kubota too.
Either machine will do a fine job on your 6 acres.
With, or without "Live" PTO, a little thought and adaptation to the machine's capabilities, you'll do a fine job mowing.
By the way, the new 790's are 27 gross HP and 24 at the PTO. Plenty of power to run a 72" finishing mower or a 60" rotary cutter. If your brush cutting is light, I'd even prioce a 72" cutter instead of a 60" unit.