Anyone who buys the $1995 Drysdale's/Challenger "Start Your Own Business" package receives as part of the package a 6" orbital stair tool, based on an automotive polisher/buffer. It is the same one that I have been using for about 3 years. Despite its light weight and small motor, it just keeps on running. I also decided recently to purchase one of Challenger's stair tools. At somewhere in the vicinity of 15 lbs., it is decidedly heavier. (Challenger Pad Systems sells it for $298. I'll sell one to any readers of this Forum for $250.)
Which is better? Each has its relative strengths and weaknesses. I've recently tested the Challenger tool on some jobs with a lot of stairs, mainly 2 large, 19th century intown Boston condominium buildings, 5 floors in height, with no elevator, and between 150 and 300 carpeted stairs. With no elevators, everyone uses the stairs. The lowest flight is always the most soiled; the top flight (logically!), the least soiled.
What I've found works best is doing the lower 2 floors with the Challenger tool. Its extra weight seems to do a better job on the more heavily soiled stairs. I just guide it from side to side, and over the lip, and it does most of the work. To clean these stairs with the smaller, lighter tool requires more pushing it down into the carpet fibers. But using the big Challenger tool for a long time is definitely tiring. Cleaning each stair isn't so bad, but moving it onto and off of the pads and stairs becomes tiring after a while on a job with lots of flights of stairs. So the compromise solution, I've found, is to use the heavy tool on the lower levels. Then, when my forearms and shoulders are beginning to tire, I switch to the small tool for the higher-up flights, which clean well & easily with the lighter tool. So now there is a choice: A, B, or A & B.