Position control is a system that allows you to set your implement (like a box blade or back blade) in exactly the same position (height) every time you drop it. It can then be raised or lowered in exactly the increment you ask it to by moving the lever a little or a lot. When the hitch gets to the position you have asked it to, it stops there until you apply another input at the lever. A quarter inch valve (like on the 7800) is either full up or full down, until you stop it by going the other way, with one exception. You can alter the height a quarter inch at a time. I have used both, and like the vast majority, find PC to be a far superior system. With a quarter inch system, there is no way to set the implement at the same height every time. Think about spreading loam to a desired thickness and which would do the job better. I use it with my rear snowblower to blow a portion of my lawn where I park my snowmobile trailer. I set the height, never dig up the lawn. You get the idea.
A three point with position control and top'n'tilt with check valved cylinders is a great tool. I love mine.