I have one of the 557's. Awesome sight - with a few caveats I wish I'd thought of before.
The bullet drop compensator sounds awesome, and it is, /IF/ you have magnification. Without magnification, you end up with a cluster of dots running down the middle of the circle reticle. You can't realisticly use the drop functionality without a 3x or 4x zoom.
The NV settings works great with my PVS-14's, but it's impossible to fight in that setup without just spraying everywhere. I'd insist that it's designed for a weapon mounted NV sight.
Unless you are fighting in the dark, and you are going to magnify - I would get the 516