This is just my opinion, try to get as much weight as you can pull with the blades axle deep, maximum angle up the steepest hill that you are going to disk. What that is in your case, I have no idea. I know that I have an old(restored) John Deere-Kielfer 8' offset pull disk, weighs 2000+lbs. I pull it with a 75hp, 12,000lb tractor and there are times that it is all that the tractor will do to pull it. If you don't care about having to lift your disk, then get the 8 footer. To be honest with you, without knowing more about your conditions, I would probably get the 7.5' Howse unit . The price is right, you'll have 55lbs per blade, and it's wide enough that it won't take you forever to get an area disked up. I doubt that your tractor would pull the 8', 1850lb unit in the conditions that I described. That's 92.5lbs per blade plus all the drag, it would be a real load that's for sure.
I don't know how much disking that you have done, but it makes a HUGE difference on the moisture content of the earth and how often you disk the ground. Are you doing this to earn money, or what? If for money and you don't know what or when the schedule might be, then you might be better off with the 6.5' unit and the extra weight per blade. At 100lbs per blade, it should do some serious tilling.
Good luck, here's a pic of mine.