Both of those gate openers are 24VDC systems that have adjustable limit switches in the open and closed position. They are very similar in stroke length as well (stroke length is the length of the screw drive that the shaft of the gate opener runs on. The longer the stroke length generally means the more leverage the gate opener will have on a gate).

Differences:

The E-S 500 uses magnetic limit switches that are adjustable by sliding them with your fingers. The E-S 1600 uses mechanical limit switches that are adjustable by turning a screw. The E-S 500 has the advantage in the longevity of the limit switches over the mechanical ones of the E-S 1600. Since magnetic limit switches do not have parts that touch or move there is less likelihood of the switch breaking. The E-S 1600 has the advantage over the adjusting of the limit switches. While the hand sliding of the limit switches on the E-S 500 is very convenient it is tricky to make slight adjustments. The E-S 1600 can make a slight adjustment very easily by turning a screw.

The E-S 1600 has some advantages for installers. It is UL rated (both meet UL standards but the E-S 1600 has paid for the certification testing) and it is more repair friendly. The UL certification helps protect installers from liability on commercial applications. The E-S 1600 housing is able to be opened completely by removing bolts and every part inside of the opener can be accessed and replaced. If an installer is offering a maintenance plan this would be an easier route for repairing the opener. The E-S 500 has some parts that are replaceable but they are not as easily accessed.