@AEO - if it was close, you could always try Kool-stop's thinline pads, they're made for such situations.
Other than that, one consideration when choosing a canti is wide- vs. low-profile. Many people prefer low profile in the rear because of heel clearance. Long wheelbase frames (like mine) can tolerate wide-profile cantis on the rear, but a lot of modern bikes can't.
Otherwise, the only other major consideration is you want a set that take threaded pads. The CR720's by Tektro mentioned are pretty good, I have them.