Is a single pair or a multi-piece set better value?

It depends on how varied your work is. A single classic 10-inch pair (like the Irwin 102L3) is inexpensive and covers most general gripping and clamping. If you regularly switch between sheet metal, pipe, and tight spaces, the 3-, 5-, and 10-piece sets carry a far lower per-tool cost and add long-nose and curved-jaw options.