We see other tandems break 10-speed chains when shifting out of the saddle. We never see 9-speed chains breaking. Chain life depends on conditions. We are lucky to get 1500 miles on our 9-speed. I get about 3000 on my single, makes sense. We ride in all weather. Team wattage also makes a difference to chain and tire life. Tires scrub off a lot faster at 22 or if you do a lot of hard climbing, as TG has noted. Other than the occasional break, 10-speed tandems don't seem to go through chains terribly faster than 9-speed. I've always gone through chainrings faster than cassettes, even with that frequent chain replacement. We also go through an average of a rim/year, V-brakes.