If you are casting big .44 or .45 bullets using a 4 or 6 cavity mold, definitely buy the 20 pound. Even if you are casting .38's, you will probably suck a 10 pound dry awfully quick, and then have to stop until you melt a new batch, and by that time your mold will be cold again.
Buy the 20 pound: when casting operate it full with the temperature control way up, and be continually feeding it small ingots. If you pause in casting, turn the temperature control down.