My cent and a half would be, well it's up to you. I looked long and hard at the AHS and Axeman-Anderson, really wanted an EFM 520 but bought a used KA-6 and am very happy with it ever since. A few growing pains of my own doing but she is running trouble free now. I still say the most important things about a coal stoker are good coal, proper installation, and proper setup ie. adjusted properly to spec.
The Keystoker is mechanically very simple and I could see the stoker mechanism running for 100 years without wearing out. It is slow moving and just a couple of pieces moving. The weakest link may be the $60 Fasco secondary blower which is a late addition to the design. However this blower could die and it would not greatly affect performance nine months out of the year. They do create a good amount of fly ash that must be periodically removed from the stack and insides of the boiler. Strong points would be initial cost and simple design, it should never break down.
The AHS is like the Axeman Anderson in many ways with what are considered a few improvements. It is my understanding they more or less self clean, they no doubt get more BTUs out of a pound of coal than the Keystoker. They are more mechanical than the Keystoker and I have no Idea what the reliability of the moving parts is. The potential for more breakdowns and costly breakdowns is there but it is just a potential. The AHS owners on the forum will have to let us know if there are any issues like this.
In the end the AHS would not have fit in my basement because I have a low basement too. The man who built the house a hundred years ago got tired of digging when he hit rock and I got tired just thinking of digging another foot or two through rock. Also the price trumped everything else. New boilers are not cheap and if you find a good used one, any make as long as parts are available, that will be your best bargain.
No matter what you end up with in the end you will be happy when you see how many dollars you save in a year.