Well, since I think I'm the only one on the planet with both, I guess I had better chime in. First of all, this isn't really a fair comparison as the SS 5-20 is a much more expensive scope. The initial promotional group buy with SH kicking some in has the SS priced at an insanely low price for the next 20 days or so. It'll be a more fair comparison when its price goes back up to the $1500 range.
I like the 6-24 PST a great deal and feel it is about as good as you can get in the sub $1000 range for a high power FFP. Very good glass, reticle, illumination, knobs, etc. A very nice scope.
But there are some things you get with the SS. The glass is better. Much bigger FOV on the low end if you do any work up close, which along with pretty equal resolution on the high end makes it a bit more versatile. A bunch more elevation travel. 10 Mil per turn knob. I like the knob feel quite a bit better. Longer/easier eye relief, especially at high power.
The PST is lighter if you're trying to save weight, but the SS has a more confidence inspiring "heavy duty feel" to it, if that makes any sense—more "Nightforce-like."
The PST has a ZS but the SS doesn't need one as much with the 10 Mil knob.
Some will probably like one reticle better, others will like the other better, items like that are mostly personal preference.
All in all both are very nice scopes and will get the job done well for most applications. Personally I feel the 5-20 SS is enough better in enough ways to easily justify the price difference (at regular price, at the group buy price it's a no-brainer).