Either belt should work fine, as long as you adjust the number of shims around the pulley. Most people don't know how to properly tension the belt (it is *very* confusing for someone who hasn't done it before), and the opportunity for error is high. Turning the engine over slightly after initially placing it on there is key. 101 Projects, and this tech article explain the process:
http://www.pelicanparts.com/techarticles/911_fan_belt/911_fan_belt.htm
-Wayne