[tipsy-]

I'm slightly biased of course, but Javalin is just a lot simpler to use. That might be because it's inherently simpler, or because Ktor's docs (as you pointed out) are very incomplete. I've tried to setup simple apps in Ktor to compare how it feels to write apps in it compared to Javalin, but I have given up on a lot of them because of the lack of documentation.

Other than that, Javalin is aimed at java-devs who have switched (or are interested in switching) to Kotlin, and works equally well in Java and Kotlin. Ktor is probably a better fit if you're a Kotlin purist.