A:Answer Well, there is no D5400.
Nikon used to update the D5XXX line every year, back in the D5100, D5200, D5300 days. They skipped D5400 (which would have been the 2014 model) but that 's not unusual, the Japanese often skip "number 4" stuff. 2015 brought us D5500, which is a dang nice camera. It has a lot of improvements over the D5300: better high ISO (low light) capability, more shots on a charge, touch screen, eye sensor,
The 2017 model, D5600, was just announced about when you posted, and there's virtually no chances over D5500. I think the only addition is NFC, so tapping the camera with an Android phone will send the phone directly to the play store page for Nikon instead of making you type the word "nikon" in the play store search window, and Bluetooth, which really doesn't bring anything to the table.
Get the 5500. They're on sale right now.