The read/write speeds on the 850 EVO are going to be higher both random and sequential, so that's what you would be paying for. Whether or not you'd notice a difference, I can't say for certain. I'd say if you are going from a HDD to a SSD you'll be happy with either one, so go with what your budget allows. You probably will be able to tell the difference only if you are paying really close attention, so ultimately go with what your budget allows and don't make compromises somewhere else to get a higher end SATA SSD.