I looked at the EFM boiler web page, looks like an older design, but probably built like a tank. Its minimal firing rate is 2 1/2# per hour, which is higher than my house would need. The Harman stoker can fire down below 1# per hour, which will save coal during the warmer weather.
The Harman stoker is a fairly beefy design for a carpet bed stoker, a PSC motor, with needle bearings, driving a ball bearing roller into a steel yoke, that rides on heavy bronze bushings. I'm not saying it can't jam, but it seems stronger than other plate type stokers mechanisims.
Both the Harman and Keystoker units have very low flue outlets, also, the Keystoker has a double baffle design, the Harman, a single, but with 8 water tube going across the top of the firebox.
I hope to see a Harman in the next couple of weeks, I would love to compare it to the Keystoker, but don't know who in North Jersey deals for them. For the Keystoker, I would go with either the smallest unit, or the next one up. If anyone has pictures of the smaller Keystokers, it would be helpful.
Thanks again for all the replies.