Some interesting choices there..
I like the 650 if the 25" tires are the new HDI avenger tires rather than goodyear rawhide III's. It will also have the high output charging system that will charge at an idle.
The 580 is 18hp, the 650 is 23. The 580 model has stock 24" frontier tires I assume? The optional skid plate and winch is nice. $8,000 or less is nice. If the bigger tires are available, I might be concerned about what ratio transmisson the 580 had. Not a deal breaker, though. The 580 doesn't have axle bearing extensions, not needed unless you plan to run certain types of tracks.
I wouldn't consider the Admiral trans a necessity in the 6 wheeler, it turns easy enough with the standard trans.
You might drive the 650 and 580, and see if the 580 has enough power to make ya happy. Either engine can be modified to make lots of HP..
To Invent, you need a good imagination and a pile of junk. (Thomas Edison)