I have 11 acres and the 790 does great.
I think the 790 is the better choice for you, for example today I mowed a neighbors very overgrown horse pasture (very dense 3 foot weeds, cattails,etc). I used a 5 foot rotary mower and maintained 5th gear or 3.5mph without any drop in rpm's or bogging. 6th gear or about 5mph was possible on the lighter areas. I bet the 2210 is great but I think doing 5 foot passes at a decent speed would not be possible so the job would be much longer and more of a workout for the machine. If I drop down to a 2210 and 4 foot mower the job would still take 20% longer. As far as gear slowing me down or being a hassle, after the first few weeks yuo don't notice it and you get pretty quick. I find I can cut as quick as the terrain dictates, the real slowdown is looking out for mower wrecking obstacles /forums/images/graemlins/frown.gif
The hardest thing to rotary mow is the thick overgrown pasture with that wirelike grass. From my experience the RM's breeze trough "woody" material like thickets, thorn bushes and small saplings, the real power hog are those thousands of strands of long wirey grass. If this is what you plan on cutting the bigger tractor is important.
I think the 790 will also give you a little better ground clearance, and a wider base to straddle ditches, etc. If my property was more "lawnlike" I probably would do well with a 2210. As it is, I cover some rough terrain and I think the larger wheels, etc. help.