Does the SS 20x have mil adjustments with mil-quad reticle? Mil/mil is always nice, but if you are getting by just fine with the mil/moa Nikon, that isn't much of an issue. The SS 16x is mentioned quite often as being noticeably dimmer than the famed SS 10x; the 20x even more so. I think magnification above 16x really benefits from 50mm and up objectives, as the exit pupils just get small in a hurry. The SS line are great, but not quite good enough glass for being fixed at 20x continually. Some guys like them fine though, it all depends on your eyes and your expectations.
If you are dedicating your setup to 600-1000 yards, it would be worth it to save up at least $700ish for a nice upgrade.
If the Nikon is working fine for you, I don't see the SS 20x being much of an upgrade, as you'll want to upgrade from the 20x fairly soon anyway, IMO.