"Supplement" just means "something extra you add". A ration balancer, very specifically, is a feed that contains a combination of the essential vitamins, minerals, essential amino acids along with either a modest or a fairly sizeable amount of protein. Fed at the rate of 1-2 pounds per day, it has a substantial calorie count as well as the typical vitamins/minerals, etc.
If you mean to compare it with a garden-variety vit/min supplement, you will find that those are minus the protein and usually the essential amino acids, and are usually fed at a much smaller volume of 2-4 ounces a day. Virtually no calories.
The main difference is the protein/calorie content.
I use both, actually. The normal-metabolism horses and harder keepers get the (Purina) ration balancer, and the easy keepers get nothing but the recommended small serving (it winds up being 3oz, which is how much a handful is when I do the scooping!) of a pelleted vit/min/AA supplement, which happens to be made by Uckele.
Basically what deltawave said. My fatty fat QH gets the 3oz scoop of vitamins...and nothing else, to his never-ending dinner time dismay. The hard-keeping TBs both get the 1lb scoop of RB (which gets them their vit/min plus 1500calories) plus whatever assorted grains they're on.
I use the Poulin MVP, have had fantastic results with it.