The spacer is stronger than the crush sleeve also,everything is bottoming out against something solid instead of just against the "squish" of the crush sleeve.
The Dr. Diff one is suppose to be cheaper and machined more correctly. The Summit one is sometimes too thick where you don't need any shims with it and the spacer needs to be machined a little also. When I did mine ,I needed no shims at all,the spacer came out right by itself so I would say that it is true.