I think you will find very few complaints from the 790 owners. You should compare not only HP and PTO HP but FEL and 3PT lift capacities. Some may feel the 790 is a little light in that area. My 1998 770, (the last year before being replaced by the 790) has been a very reliable and easy to operate and maintain tractor. (The main difference is just a couple of HP less on the 770 than the 790). As far as lift capacity, the 770 handles a full bucket of wet sand without any problems. Even with loaded tires and a 54" box scraper in back for balast, I wouldn't want to lift much more weight than it does now, so greater lift capacity is a non-issue for me.
You absolutely need to drive both and see how you fit and how they feel. I have no problems with interference between the FEL and 3PT controls. They do not interfere with each other on mine. I also like the location of the FEL controls better than on some Kubota models. The FEL lever falls right to hand without a reach.
If you're on the fence after driving both, the dealer's service reputation may be the deciding factor.
Another option might be to buy a larger tractor for the 20 acres and a decent Garden Tractor to mow the lawn. AG or Industrial tires can tear up a wet lawn, and turf tires might not provide enough traction in the fields, even with MFW (Mechanical Four Wheel Drive). (It sure is easy to spend someone elses money).
Also, I believe the Mid PTO on the 790 is an option.
Good luck on your purchase decision.