What is rack and pinion vs. chain driven slide gate opener?

A chain driven operator has a chain that attaches to both ends of the gate using a bracket. The chain runs the length of the gate and is feed through 2 idlers and a power driven gear. The advantage of chain is that it is very easy to install - especially on round tube gates like chain link. The disadvantage of chain is the fact it is noisy and usually leaves a line of grease under it across the driveway. 

Rack and pinion gate openers have a large following in Europe. There are sections of rack that get bolted to the bottom part of the gate forming a straight, level bar sticking about an inch off the side of your gate spanning the length of the gate. On the bottom of this bar there are teeth that face downward - these teeth interact with a mechanized gear on the opener that moves the gate back and forth. The advantage of rack and pinion is that once it is installed it is quiet, highly reliable, and requires little to no maintenance. The disadvantage is that installing it at the right height on the gate and level takes more time and consideration than chain. It also may require a modification to your gate.