Holosun HM3X cons:
Warranty (or lack thereof). Things have been known to break (flip-to-side: rare, clicker assembly: rare). Lack of aftermarket footprint/mount compatibility, have to use a straight riser.
Vortex Micro 3x VX-3M pros:
Aftermarket mount compatibility with Unity FTC Omni. Warranty par excellence. Stock mount is snappy. Unlikely to break (but it still can, just 50% less likelihood vs Holosun).
Vortex Micro 3x VX-3M cons:
Unforgiving eyebox. Glass quality not as great value per money. If using aftermarket mount, stock mount + risers goes in parts drawer (wasted investment).
If they sold Vortex Micro-3X without the stock mount (but with aftermarket mount) -- I don't know, team up with Unity or something and sell a package wholesale -- that would be a much more compelling option.
For cheap get a Holosun. For expensive get a EOTech. Vortex is.... just in the middle. (I own a Micro6x, not Micro3x, but slapped it on a UH-1 with FTC + Unity riser.)