I have both and here are the differences to me: Sightron is clearer, but not by much over the ED which is clearer than all my other scopes. ED ranges slightly better than the SIII at 16X, SIII is lighter by about two ounces, ED reticle is thinner and sometimes hard to see in very dark lanes/targets, but the illumination solves that problem, therefore the SIII reticle is easier to see and its reticle illumination also works well for dark lanes. The ED scope is Mildots and my SIII is a MOA-H, which is MOA sub tensions. The -H stands for Hector and unlike the other SIII's, the entire reticle lights up not just the center dot.
The SIII doesn't come with a sun shade, $50 extra or make one from a cardboard tube, much lighter too. There is a website called www.DVOR.com which is a clearing house for Optics planet which has good deals on those scopes. I got a Hawke 10-50 ED scope there for $599, they aren't there all the time but do show up eventually, same with Sightron/March/Nightforce/Leopold scopes. Have seen the SIII's there for $1000, but some have minimum focus distances of 13 yards and some have 9-10 yards.