Which brings up the question; "if you're using a 6" polishing pad on a 5" backing plate which weight should you use?"
There are two main answers, the first being; "whichever feels better to you." But a number of Autopians report it as; "it doesn't matter, we can't feel any difference."
Keep in mind the 5" and 6" counterweights were really designed to be used with Porter-Cable's sanding plates and abrasive disks, not polishing pads and other manufacturer's plates, so they're more a suggestion than anything else.
If you get both play with them and decide which you prefer. Otherwise, I wouldn't worry about it too much.
PC.