Depends on which lenses you have. The D90 has a focus motor and the D3400 doesn't. That means that older Nikkor lenses auto-focus with the D90 and those lenses won't auto-focus on the D3400
You'll gain some features, you'll lose some features...
You'll lose the top-mounted LCD, and the AF motor for example; but you'll pick up some dynamic range, color depth and battery life. And of course you'll have twice the resolution of the D90 in the D3400.
That's a good choice. I believe the D3400 is menu driven - meaning to make changes, you have to go through the menu on the back. The D90 and D7000 have the shortcuts with the wheels and buttons for many setting changes. Have fun with it!