Cdk8s is better at generating yaml. Pulumi is more of a terraform replacement that has a good kubernetes provider (the tf one sucks). Though it can generate yaml as well it is really not designed for this.

IMO there is something nice about having all your infra, both inside and outside kube, controlled by one thing. Then you can declare a whole application in one rather then half of it in one tool and half in another.

I would prefer pulumi as this would allow me to spin up the entire infra and manage k8s from it.

do note, js/ts typically are tier 1 language for aws cdk and pulumi.