The 512 is the least expensive model and you get what you get - a huge form factor because of the AA batteries that doesn't leave a whole lot of room for a buis. When you add a buis, it's absolute co-witness only. That is why Eotech made the 518 - they raised the sight up to get the lower 1/3 co-witness, put the buttons on the side (you know, so that you can press them if you put a buis or magnifier behind it), and gave it a QR attachment (so you are not always wrenching on it with an allen or that thumbscrew to confirm that it's not sliding around).
I don't consider myself a brain trust, but if you are sold on Eotech, I would recommend paying the extra to get the XPS or EXPS ones with the transverse battery. The shorter size gives you more options in terms of where it can go and the accessories that can go around it. Unless you actually have NV gear now, don't pay the premium for NV settings that you will literally never use. While the newer Eotech's may be better, they still burn batteries in comparison to other technology since it is powering a laser, making a holographic image, not a LED. Always have extra batteries in your shooting bag. It's a bad feeling when the recticle starts blinking as soon as you pull it from the case.