salt - no steel liners inside the handle, 2 positions of the clip (only tip up, the Endura 3 style... i think), thinner blade,
extremely corrosion resistant and unfortunately soft blade steel (you need to sharpen more often at least in the beginning (Spyderco and users say that it gets better with time as the steel is precipitaion hardening type - use and sharpen more and it gets harder and the edge will last longer. The salt is a bit shorter compared to endura and the jimping at the back of the blade is less agressive.
Endura - big, solid, 4 clip positions, 2 steel nested liners (higher weight), VG-10 (long lasting edge, high resistance to wear and corrosion. But not that corrosion resistant as H1. Tests show that H1 cannot rust. This is not a joke), thicker blade.
Overall the E4 wins if you search a tough worker, and Salt if you work in high humid environment.
One thing to add E4 is saber grind and Salt i think uses hollow grind.
Others will add or correct
