Same as with your filter question... the "correct" one is the one with the features you need. How much power, perhaps the ability to work with Nikon's CLS system, which the D5600 can't do by itself (it needs a compatible flash or controller on the hot-shoe to control off-camera flash units.) Generally speaking, the higher the model number of the Nikon flash, the more powerful it will be. All of the Nikon flash units feature fully automatic flash exposure capability, and even automatic fill-flash (using the flash in an otherwise brightly lit scene to fill in shadows.) More $$$ is basically more flash power.