The xps series is a newer more well thought out design. They use the shorter CR123 batteries, which are pretty easy to find online for only a couple bucks each. The batteries are mounted laterally so that recoil impulse won't cause the batteries to lose contact and reset the sight. (a common problem with 5 series models).
The only problem is, I don't like the way the xps looks, the 512s look alot nice.
One piece of advice. Please try and handle an eotech and see if the reticle works well for your eyes. My eyes (astigmatic and lasik'd) can't handle the Eotech reticle.
Assumption is the mother of all ****ups.
Know what you know, Know what you don't know. -Paul Pedzolt
"Jack of all trades, master of none, though oftentimes better than master of one" -Mynshul