Not to the same scale, the MoonLite Crayford Dual Rate/Filter Swing model CR4 focuser at the left is a Low 1.45" profile variation of the Rack and Pinion focuser. This variation replaces the pinion with a shaft that has a sprocket wheel on it to engage the rack on the 2.78" draw tube. The draw tube rides up and down within the housing through two sets of ball bearings. The advantage of this is a smoother riding draw tube which translates into smoother focusing.
It is also possible to replace the sprocket wheel with a friction wheel that will engage a smooth rack under tension.  This modification allows for more accurate fine focusing.  Here, dirt between the drive wheel and the draw tube's rack could cause the focus to slip. Some drive wheels could be warped out of shape by constant pressure against the draw tube, giving them a flat spot that would make their focuser useless. It really is the durability of the materials that makes the difference. The $365.00 US focuser on this page is not likely to suffer from such problems because the manufacturer uses good materials. All the metal bearings are highly-polished before anodizing.  The Dual Drive feature provides the second focusing knob with an 8.2:1 gear ratio making it easier to get a good fine focus.
Note the swing out filter holder on this model. Filters are among Astronomy's most useful telescope accessories. A Friction Drive is a natural for Electric Focusers. Under electronic control, such focusers can be used from a remote location, and can be automatically adjusted to compensate for the changes in ambient temperature. This is a tremendous advantage in Astrophotography when taking photographs through an eyepiece.