Betting may drive out a bigger ace (thereby earning you 3 more outs), or may earn you a free card on the turn. If people are just calling your bets (and you are not overly aggressive in general), then they may be drawing to the same hand you are, and will fold even if they manage to hit a low pair on the end. Betting is usually a better option because it means you have another way of winning if you miss, and that's when everyone folds.