The retainer on the 15 is slightly bigger than on the 15HG. The slip ring on the 15 is flared and is a regular cylinder shape on the 15HG. Lastly, the "O" ring on the 15 is rubber and the "O" ring between the locking nuts on the 15HG is Delrin.
Hope this helps. I always like to try new things, so my favorite is the 15HG.
Ron - H&H