the design on the left side. looks stiffer and more stable compared to the one on the right.
but the one on the left will require a larger stepper motor or extra gears to produce more torque, then the other one, since it will carry the extra load of the 2 shafts the stepper motor and its mounts, the treaded rod and its assembly. while the one on the right side doesn't.
"edit" sorry point already made. lol
http://www.putfile.com/powernaudio "some of my car audio install page ",
maybe a CNC router will make my audio enclosure builds allot easier. lol