Smaller tubes every time. Less chance of popping them if you're mounting a tire with a tight bead and less chance of a pinch flat when riding. I just installed 27mm tires with a 18-23 tube. The tire is a Challenge Paris Roubaix open tubular with super tight beads. I popped an 25-28 tube three times and gave up. Got some 18-23's and they went in no problem.