M has a zero turning radius to get out of tight spots. No design and software problems as with steerable front wheels and big turning radius as found on RC cars. And S . M . only uses the skid buttons when stopping or starting. With the load balanced, S .
M travels only on two wheels.
M . travels over almost all indoor surfaces with no problems. Elevation changes of up to a half inch are no problem. If your current robot project already includes wheels and modified servos, just move them over to the S . The best wheel size is 3 inches. Or just buy the wheel and servo kit from Norland Research.