Here's my .02.
I have driven a '79 6620, an '84 7720, and 2 different '87 8820's.
The 6620 was bought new and we ran it until '03. We had a 20' flex on it and it was all that it wanted. Good overall combine but final drives were the weakest problem even when we were running a 4 row wide and a 6 row narrow. Didn't have enough power to my liking. Always seemed to choke down which I hated. Though it was way better than the 6600 that it replaced, by far.
My uncle has the 7720. I drove it quite a few times and really liked it better than the 66. Drove better and handled the 24' flex fine. He runs the same 6 row narrow on it and it could handle another 2 rows without a problem. It had been well cared for by the previous owners and it ran like new when my uncle first got it. I believe it barely has over 3000 hours on it now. If both the 66 and 77 were in the field at the same time, I was trying to get behind the wheel of the 77. It's that much better. IMO.
We now have an 8820 Titan 2. I started out on the 66 first, then drove a 77, and then finally in the 88 and I am going to tell you,....... there's NO WAY that I would go backwards. The capacity, the power, the way it drives (especially now with the duals) is just uncomparable. They are all in essence the same, but with the 88 you get that much more of a machine and haven't had a minutes trouble out of it since we got it back in the summer of '03. It handles the 25' flex easily and could take a 30' without any problems. The 6 row narrow was WAY too small for it. It wasn't loading the machine at all. Now we have an 843 and it still has no problems with it. It could handle a 12 row without a doubt. The final drives are planetaries with axle spacers and the axle is so much bigger than the 66 we had.
When we got this 88 back in '03, we teamed it with the 66, side by side. My granddad in the 66 and me in the 88. That thing was broke down all the time that fall. It was irritating to my granddad. He was trying to keep up with the 88 and he just couldn't. So what does he do? He goes out and buys himself an 88 in Jan of '04. Another '87 model like ours(mine and dads).
My granddad always said that the 88 was too big for our operation, thats why they (him and my dad) bought a 6620 back in '79. And after all those years, he saw first hand that the 88 was one helluva machine and bought one himself. Just wish that he and that combine were still around.
Go with the 8820 Titan 2.