Never used the Sig.
It's been a few years since I've looked through a Strike Eagle. After looking through the one I bought, I didn't even mount it. I just sent it back and bought a Trijicon Accupower.
I recently bought a PA SLX 1-6 for an AR10. I bought it really because I wanted to see the Nova reticle. I have been impressed with it. Don't get me wrong, the glass isn't amazing, but it's really impressive for a $300 optic. The Nova reticle is awesome for close to midrange distances. It truly is like shooting with a red dot sight up close (minus the bloom) and you still have to ability to reach out a bit or for positive identification.
Edit. Saw the above poster mention Swampfox. I tried one of theirs only because Michael Branson was working for them at the time. It was also very solid for the price. I would get either of these before a the Vortex. Vortex makes some nice optics on the upper end. Yes, they have a great warranty. Yes, everyone knows about it. The reason they know about it is because so many have had to use it.