The power difference is negligible, and both use "power-packs" vs discrete output transistors. Both sound pretty good for "mass market" amps. The price seems very much on the high side for either, unless they have been recently serviced and are in very good cosmetic condition. If so, and if you are willing to pay at the higher side of the market, choose the one that looks best to you. Note the unusual rear panel of the SX-650...some folks like it, some don't.