I have 31.8 on my cross bike, and 25.4 on my mountain bikes, both are fine. A couple of potential issues with 31.8 IMO, mounting a lot of stuff on the bars might be more of a pain, especially with a riser, think of trying to mount a light, computer and hrm, might be tight. There was a test in the German road magazine Tour on frame comfort, long story short there wasn't much difference between frames of all materials in comfort, but there WAS a noticable difference between 26 and 31.8 bars. According to them the 26 rode much smoother.