9200 Pro - higher core/memory frequency (275/550 respectively, as opposed to 250/500 on the regular). The benefit is anywhere from 2-5 fps, though the only reason I suggested it is if there is a very miniscule price difference between the regular and the Pro.
9100 - Operates with 4 rendering pipelines with 2 texture mapping units per pipe (as opposed to only 1 on the 9200) - in plain English, games which utilize multi-texturing (laying more than 1 texture/texture element map on a given surface area) will defintely benefit from this (resulting in higher performance), especially FPS games such as HL2 - only "downfall" to the 9100 is lack of AGP 8x support.