Some of the design differences:
Superlift 4", Skyjacker 6" - both leave the rear as a spring under (SUA); you must replace the rear springs. Superlift 4" springs have been notorious for sagging. Superlift 4" uses the exact same drop brackets as the 5.5" lift - just 1" higher holes are used.
Superlift 5.5" - spring over in rear (SOA). You get to reuse the rear springs and can reuse the rear brake line if desired. Cheaper. If the springs do sag, it's an easy fix (stock springs) Some people will argue SOA is better than SUA. Hey, I like SOA, for it gets *everything* up above the axle.. less stuff to drag!
Superlift 5.5" - there has been 3 redesigns of the drop brackets. 1st design was a steel plate, and re-used original bracket (moved it down). 2nd design was a c type bracket - 3rd design is a built up bracket, welded and stamped. Biggest problem - no reinforcement for the bracket, so both the cross member and the bracket bends (and eventually cracks welds). (Go look at a Rancho 2.5" lift, and you'll see a great design, just too low for some of us..)
Radius arms:
Superlift's are built up from 3 pieces of steel - they also lay another 2 peices on the ears now (if you have diamonds or a single layer, yours will crack; call Superlift ASAP to get new ones). They also move bushings back to about the transmission cross member. The bushing mount is easy to catch, crack, and rip apart (been there, done that). I'd like to see them built up out of tubing..
Skyjacker radius arms: Uh, I'm still waiting to hear how the are constructed (photos would be nice too!).
Springs - Superlift just issued new coil springs for the front - came to be, they was using ranger springs, and the explorer (esp. the 4dr) was heavier. They have replaced the old coil springs with a high spring rate coil.
Skyjacker 6" springs are softer than superlift 5.5" springs (I've heard..)
Lower spring rates mean faster sagging.
Superlift also has the Superrunner steering kit; makes a big difference on Bump Steer. This kit can be used on any explorer with 4" of supension lift. (won't work on anything less).
As for installation, I don't see how they could differ; both kits have you remove the old drop brackets (time consuming), remove and change the radius arm bushing locations, and/or replace the radius arms themselves, and change the pitman arm out.
Superlift 5.5" you only have to disconnect one side of the rear springs, roll the axle out, roll axle back in, place SOA pads on, and you'r basically done.
Superlift 4" and Skyjacker 6" you have to remove the rear springs, and replace them with new springs; you also have to replace the rear brake line.
Any other differences? I'd like to hear them... (facts and pics please!)