If what you're after is as much power as you can get in a small package, Sony's A6600 is a very tempting offering.
Real-time tracking and AF is fast and accurate; In-body stabilisation; 4K HDR video capture; Longer battery life
Flip up screen design isn't great; No 4K60p (but there is 30p; Quite pricey; Many of the features available on the A6400