Hi, I have used both but the choice really depends upon the usecase as JobRunr supports both adhoc and cron based jobs similar to kubernetes. Here are some datapoints for consideration:

1. In certain circumstances, a single k8s CronJob can create multiple concurrent Jobs. But in case of JobRunr they guarantee single instance of the job
2. I would chose JobRunr if we want to standardize on a job scheduler platform that can run across different container orchestration system (like ECS, Kubernetes, docker swarm, etc) thats platform independent.
3. Even though its easy to get started with k8s cron jobs it can quickly become complex when coordinating between two jobs, data sharing, etc. So if your usecase is simple you can go with k8s cron jobs, but if you think you will add a lot more features to MVP version of your app related to scaling and concurrency then I would pick JobRunr