from what i have been told, the treo ssi will handle 1000wats rms although it is rated at 750 or 800. and the rl-p is rated at 500w rms(conservatively). the price difference is $20 with the treo costing more. the rl-p is best used in a smaller box though, 1.5 cubic feet i think, so if you are limited in space the i would go with the rl-p.
i have 1000wrms going to my one of my treo ssi and its asking for more. there are some people on another forum that powers them with a 2000wrms amp for daily use.