They are pretty close to equal by weight. Pellets have better extraction as they are chopped up. However, the process comes at a price as lupulin is lost.
I'd say about 10% less IBUs with whole hops, as stated above, not much. If you're measuring out whole hops, measure out an ounce, add like 3 or 4 extra cones, and consider it equal to one ounce pellet.