Neil,
these still bolt to the body rather then the fram as the RS correct? If so . . . doeen't impact still affect the body as with other sill guards?
Steel is more forgiving than aluminum, and it also slides over rocks better. These sill protector style of sliders are designed to dissapate impact load so that a point load impact (your truck landing on a rock) spreads across the entire sill length, thus reducing the potential to deform the sill.
The SG style sliders are tubular, so if the point load exceeds the steel's rigidity, the bottom tube face bends but the top face of the tube does not...hopefully. The aluminum wrap around plate designs, like Rasta's sill protectors, dissapate the load somewhat, but are much easier to bend. Mine looked like a piece of bacon after my testing (torture), with corresponding damage to the sill of my truck. I don't think the SG style of slider will have that sort of negative impact. The tubular sliders offer good protection from that sort of damage.
Steel plate are almost as good as tubular, because of the qualities of the steel. The RS sliders use a heavy gauge steel plate. I am not certain, but I think (Neild's?) sliders are steel, and 5mm think. I think (really not sure, old memory) RS's is 3/8" plate, which is a bit more than 9mm thick. An additional difference is in how they attach. Most plate designs, and the tubular designs, bolt only to the door sills per the hardpoints indicated in the cutaway photo I posted. RS's sliders do not. They use the pylon points that connects the body to the frame, which are MUCH stronger. This is the primary concern I have over the outrigger sidestep style of plate sliders shown on those Flickr photos. That's a lot of cantilever force to put on a little nut tackwelded to a fairly light gauge body panel.
I think aluminum plate sliders are good. I think steel plate sliders are a bit better, I think tubular steel sliders are better than plate sliders, and I know for a fact that RS's full frame plate and pylon mount sliders are the absolute best.
Once you pick your level of protection/cost/value, then you have to decide on if you want side protection via stand off tubes. It is more popular in wooded areas to have "helicopter skis" for stand off tubes, to keep those pesky stumps, logs, roots, and trees away from your sheet metal. In very rocky areas, such as SoCal, the stand off's are less helpful, and can even be a hindrence when trying to slide over rocks projecting into the trail on each side. I prefer the Stage 1's without tubes for this reason. A very good compromise is the Stage 2 Slim's, but that's just my opinion based on where and how I off road....or did :-(