What is the difference between these two Technic half bushings?
They both appear to have the same part ID despite being quite different.

Sariel answers most of your questions in The Unofficial LEGO Technic Builder's Guide.


  toothed half bush with a cutout
  
  The second variant, the toothed half bush with a cutout, is exactly
  what its name implies: a copy of the previous version with part of the
  axle hole cut out, which makes the axle hole large (see Figure 4-13). 
  
  This modification was introduced to make it easier for children to
  take half bushes on and off the axles, and that's exactly what makes
  it much less popular with builders: These bushes are more likely to
  shift when stressed than the first variant. 
  
  Other than that, it has every quality of the original toothed half
  bush except that, having been introduced a few years later, it's newer
  and thus made of strong material.
  
  (Chapter 4. Page 36.)


Pawel "Sariel" Kmiec, 2012. The Unofficial LEGO Technic Builder's Guide. 1 Edition. No Starch Press.