A long time ago, working with the firmware of a board designed to measure public electric power, I accidentally skipped one of the daily steps to setup the gig, and burned the PC (the programmer for some reason was left intact). It was another programmer for another microcontroller, but it was not electrically isolated; so unless you intend to work with environments that may pose some risk to you or to the equipment involved, there would be no reason to purchase the more expensive model.