Boy, That is a tough call. As much as I like the Clayton, and by the way Clayton did at one time produce an outdoor unit many years ago before Clayton was purchased by United States Stove Company, I'm concerned about the actual burn times since it is based on the smaller 1600 series furnace. The indoor Clayton works well if installed in the basement. But even so, during the cold weather it does need fed 3 times a day. So, will the outdoor unit provide the same amount of burn time would be my question?
The Hopsco energy is a interesting product also, but once again I'm not sure about it yet.
The Shaver is quite an impressive unit and in my opinion would probably perform better than most. I like the heavy insulation and the larger firebox. I would think that it would provide much longer burn times, but possibly use more wood than the Clayton.
So just from studying your choices, I like the Clayton over the Hopsco. I have nothing against the Hopsco, but much experience burning in a Clayton and love the firebox design. But ultimately the Shaver appears to be a much better outdoor furnace construction and would probably allow a longer interval of loading to possibly just twice a day.
The installation cost of the cement pad and insulated duct runs need to be factored into your decision also. Remember all of these outdoor units won't work without electricity either. So, you may also want to think about back up power too? But then again, that may give you an excuse to get a Chubby for the inside your house too.