I have heard that basically all "cheap" optics dont hold zero well.
Its either Aimpoint or Eotech.
HOWEVER
The Vortex SPARC just came out and is going through some testing by individuals that have the money to ruin them, and its proving very durable and holding zero well.
If you cant afford the $400-$600 for an Aimpoint or Eotech, I say get the SPARC. It comes with 3 different mounts and a 2x Magnifier.
Cant beat that
I have just started saving to get an Aimpoint H-1 for my AR, but I am considering the SPARC now.
Go to youtube and search for Aimpoint torture test or eotech test ect...
People drop them from moving vehicles, hit them with hammers, freeze, submerse in water, all that.
Aimpoints are the gold standard for durability and holding zero, eotechs are good also.
I dont like a huge optic up there, so I like the Aimpoint T-1/H-1, Eotech XPS series, and the Vortex SPARC