Just from a warranty perspective, Holosun stuff has a very limited lifetime warranty, limited to the original owner only, needs to be registered first and is non-transferable, meaning zero warranty on used Holosuns, and only lifetime for the metal structure/housing. Glass is usually only 5 year (lifetime on Elite models for original owner), electronics and LED is usually only 3 year (10 years on Elite models for original owner).
In contrast, Vortex stuff has full lifetime warranty on everything, for any reason, for any owner, at anytime, forever... No sales receipts and no registration required, no questions asked.
Personally I'd never buy a used Holosun, and contemplate buying a new one because it has zero warranty when selling it, not so good for resale, but most people are none the wiser.