The SALT gun is a CO2 based paintball gun that shoots pepper balls accurately to about 60 feet.   They advertise a 150 distance, but the lighter pepper balls aren't designed to go that distance with any accuracy.   The gun holds about 7 pepper balls in the magazine and must be loaded with a CO2 cartridge at the front of the gun before firing.  The CO2 cartridge is good for about 10 shots and then must be replaced.  If the CO2 cartridge has been punctured and you go to use the SALT gun, then the gun won't have enough pressure due to leakage to fire the pepper balls.  It's best to keep a new cartridge in the gun at all times.
Pepper balls are good for crowd control and typically should only be used outside.   Each ball is filled with a Pava powder that will go everywhere once the ball breaks after hitting the target.  This cross contamination is one of the downsides in using the pepper balls when shooting at an individual in a group setting.

The JPX 4 is the only pepper spray gun that actually shoots live liquid pepper.   The product is designed for a one on one confrontation and is fired using individual primed cartridges.   These cartridges work exactly like a bullet in a revolver meaning that they are ready to go at all times.   No worry about batteries going dead or a CO2 cartridge leak out.   The product is much smaller and lighter then the SALT gun making it easier to carry.   The JPX fires up to 23 feet accurately unlike the SALT gun that is can be accurate at 50 feet. 
The JPX offers several holsters for carry, has a built-in laser (LE model) and the pepper is an extremely hot 4 million SHU.  This makes the product very universal for personal carry, car carry, security and law enforcement.   The JPX is also ideal for School defense because it fire a liquid and there is no major issues with cross contamination. 

Both products have their uses, but for the best all around carry weapon, the JPX 4 is the winner.