When comparing the specifications for these two models, from Midway, I see the only difference is the 512 has a "Two year manufacturer's warranty from date of purchase".

There is a significant price difference. The 512 is $376.99 and the 522 is $458.99.

Does anyone know any reasons for the $82.00 difference? Is it because of the warranty?

And, if you were to purchase an EOTECH which model would you select?

The 552 is NV compatible, and has an extra button to that effect.  

Deciding whether or not that feature is important to you will determine which you feel you'd be better of buying.  

Honestly, even for military use, with IR lasers mounted on most combat weapons, the NV capability is not needed, so you not only need to decide whether or not you ever actually intend to use with night vision, and if you do, are you going to use an IR illuminator?  Or actually mount the NOD on the weapon.

HTH,
~Augee

I am not 100% sure but i think that the 520 series of eotech were intended for archery applications .

Yep, the 520 series are archery sights, with a MSRP of $330-340 - www.eotech-inc.com/arch_hwsmodels.php

For the other main difference between the 510 series and the 550 series for firearms:

Eotech 55X - waterproof to 33 feet
Eotech 51X - waterproof to 10 feet

...planning on going diving with your rifle