The reason you would get the "2x8gb ddr4 2400 mhz " is to have the ability to upgrade the ram in the future by having less ram places taken which a higher amount of ram per stick. Why would you want to upgrade the ram in the future if you are buying an i5 cpu. Imo it would make more sense to take an i7 and leave room for upgradable ram in the future since you cant justify getting an i5 to upgrade to an i7 a while later in most cases.