I was looking for a 250 when I stumbled into the deal I got on my 500. Based on what I've heard of the 250 (I've still never actually ridden one), I'm glad that I ended up with a bit more power, since I'm a bigger guy riding mostly out in the country.
I always wanted FI and modern styling, but the little Ninjas were just too much of a power drop for me to justify it. With the 400, I finally felt it was close enough to the 500's numbers that I could deal with it. On paper, it's lower on power, but in everyday riding, I don't really notice it. It feels like a bit less power if I think about it, but it also revs a bit higher. The drop in torque is most noticeable, but I've never thought, "Man, I'm really missing the 500's power" while out riding. It does feel very close, and I had intake, exhaust, carb, and ignition mods on the 500 too (though I doubt they actually added much actual measurable power). The 400 also does a better job of maintaining peak power to the rev limit. It sounds like it's working a lot harder, and it doesn't feel like it's pushing you back in the seat as much, but it somehow feels like you get up to speed a little quicker than the 500. I'm sure the FI and lower weight help.
People have said the 500 feels much bigger and less agile than the 250/300. I've heard people say the same thing going from the 500 to the 650 (though that was with the older model). I can only imagine the difference going from a 250/300 to a 650. I never thought the 500 felt that big or clumsy, but the 400 is much more agile. They shortened the overall wheelbase while lengthening the swingarm. They managed to make it even lighter than the 300. When I first got it, it really felt like it turned just by thinking about it. I still want to get some springs matched to my weight, but in stock form the suspension really isn't too bad (whereas the 500 had soft, mismatched spring rates).
The 400 comes stock with many of the mods I spent a lot of time and money putting on the 500, or never managed to work out. It has the same size wheels/tires, and floating brake rotor, as what I spent $900+ adding to the 500. The dash has a digital speedo, big analog tach with a needle that turns red at the rev limit, a gear indicator, and a clock; I looked but never found an acceptable replacement for the 500's dash. It comes with almost all LED lights, including headlights (and the rest are easy to swap). The dash has a builtin spot for a "lighter plug" for powering accessories (though Kaw's official parts are ungodly-overpriced). It has a hollow steering stem, so you can use that for a RAM mount. The mirrors work quite well stock, whereas the 500's were pretty much useless and the NewGen mirrors I swapped on were "good".
Looks are subjective. But the 400 looks like a smaller version of the new, big bikes. It's not a Fisher-Price version of the "real" bikes - it's the same parts, or the same parts scaled down a bit. If you like the new Kawasaki flagship bikes, you'll like the 400. The 500 had much less plastic on it, so full fairing coverage was a bit of a change for me. The different pieces tend to hook together and overlap, so you frequently have to take off several panels to get at whatever you're trying to do, but they're pretty simple and straightforward. I'm not sure what colors you consider good, but you may be able to get a leftover '18 (I don't think there were any major changes for '19) in a better color and save some cash too.
My biggest complaints? It doesn't have a centerstand, the same as most bikes now, but I got used to the 500 having one. The stock muffler is really close to your heel, but a
cheap slip-on will fix that. The tank is smaller, so I have to refuel a little more frequently. The windscreen is quite a bit lower - about at the bottom of my sternum vs. the 500's being around my shoulders in my usual riding position (a double bubble helps that, with only 4 screws). The clutch has an extremely light pull, which makes it easy to slip the clutch when you don't mean to (there are a few mods specifically related to this). It looks like it's still got the crappy old SH regulator, so I'll probably swap in one of the MOSFET FH models. The oil drain bolt is aimed pretty much directly at the sidestand foot. They should've put LEDs in the rest of the lights.
Going into it, I expected to
tolerate the downgrade to a 400. However, I've found the downgrade to be quite tiny, with a bunch of other huge improvements. Now I wish I hadn't waited so long to buy it. As others mentioned, there's a pretty big price difference in the bikes you're looking at. If you're going to buy the 300 and leave it alone, it's hard to argue with half the price. But if you're like me and going to be constantly trying to fix all the little less-than-ideal things, the 400 is starting in a much better place than older, cheaper bikes. Not to mention that a new bike has a warranty and no unknowns from the previous owner(s).