I have not used it, but I'll better give a poor answer than no answer 😛
Ktor is developed in Kotloin. AFAIK it supports Kotline/Native and blends a lot better into the Kotlin ecosystem than Java frameworks. On the other hand Spring is very mature, battletested and there are a lot of resources.
But if my team would not know Spring or Ktor at all, the project was not super critical and we are starting on a green field, I'd cerainly try Ktor.
Personally I know a lot of Spring, love all the features and we already have several Spring services, there is not so much room for such experiements.