Both of these models seem to have the same standard and lower speed options, only differing speed wise in the higher optional speeds.
From the order forms, I have no idea what the codes mean:
"6.0 MPH, 4M13 C60 C23" (standard speed)
"4.8 MPH, 4M13 C70 C21" (more torque? unknown)
That's all they say, so I'm not sure how they differ. If I were to guess, it looks like both options use the same ST4 (6 pole!) motor, but maybe different sprockets and chains. I'm seriously considering the 450 model for my next chair, but don't want to have to pick the 4.8 MPH option if the 6.0 MPH option will be just as good.
The faster speed options specify the ST3 motor, which, lacking details, I suppose is faster but has less torque?
My eventual goal is to find someone in the US to change it over to a Roboteq controller and lithium power, but I'm not sure how involved that will be. I can buy the pieces, but I am not qualified to do the work myself.