I only use Objectdock. I used to use Fences but I like not having all them icons on my screen without having to click it twice to hide them. You should try both of them. There are public versions of both available but the Pro versions give them more flexibility. See which one you like. Who knows...you just might wind up keeping both.
As for differences, You can have multiple Fences on your desktop organized according to the programs you use, I had five at one time. With OD you can have one dock, the free version and the Pro allows for multiple docks plus a tabbed dock. With Fences you have only the Fence itself, with OD you have various skins, called Objectdock backgrounds, that you can pick and choose from. All in all excellent programs.