The D3400 will have a (theoretically) bit cleaner image, but I doubt most folks could notice. It also has a deeper image buffer and built in Bluetooth to transfer picts to your phone.
The D3400 also ships with the AF-P kit lenses. These are a mixed bag. The lenses are quieter and faster to autofocus, but you have to go into menu settings to switch VR on/off (yeech). AF-P improvements will be most notable with video autofocus, but if you're serious about video you probably don't want to use autofocus plus you'll miss the mic input.
But here is what you give up to get that...
1.) Much less powerful flash.
2.) Ultrasonic sensor cleaning.
3.) Mic input.
4.) Accessory Terminal.
5.) In camera panoramas.
For me, 2, 3, and 4 would be deal breakers. YMMV