I haven't compared them directly next to each other. The 31 is maybe a little quieter but not a lot. On high speed, the 31 has a LOT of suction for a little handheld.
you'll be glad to know that your beloved Dyson company is doing very well in the "Reliability" ratings of North America's Consumer Reports magazine. Consumer Reports gets their "reliability" ratings for products by sending questionnaires to their membership/subscribers. The question set is usually in the form of: "Have you bought a new vacuum in the past 12 months? What brand is it? Have you ever had to submit this product for a repair in the first 12 months of use?" In both the upright and canister categories, Dyson's North American products are rated very reliable compared to other brands. If I remember correctly, the only manufacturer that ranks better in reliable uprights is Kirby!