It depends. On one hand, bagless models have no recurring costs. On the other, bags can help a vacuum last longer by keeping debris out of hard-to-clean parts that will slowly clog the machine over time. Air quality experts tend to advise that bags are better for people with allergies or asthma because they’re easier to throw out without spilling debris or kicking up a plume of dust. Bags will, however, need to be replaced several times a year, and if you have pets, they’ll tend to fill up faster, and the costs can add up.