I have the Harman boiler and like it. The Keystoker KA6 was very high on my list too. I had a Mag stoker so I was very confortable with Harman, and there was a Harman dealer within an hour of my house. I had a chance to see a Harman boiler working before I bought mine, had I seen a Keystoker, who knows?
Here are my observations and opinions about the two.
The Keystoker KA6 uses their older more robust stoker design, not the same one as the KAA2 or the Keystoker stove. It has an external gearbox tied off of the main blower which actuates the pusher bar. A small auxillary blower, a recent addition helps keep the fire going during warmer weather and helps in more complete combustion.
The Harman uses the identical stoker from the Magnum stove. It is a very robust design also, and uses a quality dayton gearmotor with needle bearings and drives a heavy pusher bar riding on two bronze bushings. The pusher itself is a large aluminum block. The Harman pushes the coal up a ramp onto the grates whereas the coalpath on the Keystoker is all downhill. Clinkers blocking the coal on the grate can be a problem on both designs, but the keystoker might be a little more suseptible to fire burning back into the hopper if the draft is poor. The Harman stoker motors tend to run very hot when they run continuously, leaving the cover off or adding a small muffin fan helps keep them cool. Are you listening Dane?
Keystokers are one piece boilers, the flue opening position has to be specified when it is built at the factory.
The Harman is a two piece with a seperate base that can be flipped fairly easily during the installation.
The KA6 is a larger boiler than the Harman with a 55 gallon water jacket, vs the 50 on the Harman, and a 275# hopper vs the 250# on the Harman. It has an 8" flue vs the 7" of the harman. It is rated at 144KBtu input, 122KBtu Net, the Harman just lists the input as 95KBtu. Both are carpet or bed type stokers and burn rice coal. The Harman will probably burn a little less coal in pilot mode, due to its smaller grate area. During the summer months my average coal useage was 12# per day, and this supplied all my hot water.
Keystoker uses two water walls vs the Harman which uses a single water wall and 8 schedule 80 pipes on the top of the fire box. They both have a large amount of surface area exposed to the fire and will probably perform very similarly. Both completly surround the firebox with the water jacket. Both boilers have a very low flue opening so heat is trapped within the boiler.
The Harman has a scraper rod that helps knock off accumulated ash off the water tubes and water wall, the KA6 doesn't.
The Harman is built with .250" steel plate, and has been in production since 1989, not sure about the KA6, but I suspect its also .250" steel plate. I'm Not sure when they started production but its probably over 20 years. The Harman boiler looks to have mostly machine made welds, the Keystokers look to be welded by hand.
Both Boilers can be fitted with oil guns for an alternate fuel supply.
You should also consider the AHS coalgun, lots of folks on this board have them and love em, a very similar design to the Axeman Anderson which is also stil lin production. Both have higher list prices then either the Keystoker or Harman.
Good Luck with your decision.