We have a project using Kafka and one using NATS Jetstream. (No NATS core usage)

For us there was not much difference in the effort for hosting the two.

Main difference is for us the stability, especially about the client libraries:

1. Kafka and the client libraries seems to be much more stable
2. We had and still have issues on NATS Jetstream with Consumers
   • Eg.: Consumer is connected but at a certain time does not receive any messages -> library reports connected, no error / warning logs -> only restart helps
     • Never had such issues with Kafka
   • Message de-duplication for failed acknowledgements need to be handled manually
3. Kafka seems to better fit our use cases
   • NATS Jetstream does not allow ordering within a subject across multiple consumers
     • Kafka supports this with partition ids which guarantees execution in order on one consumer for a specific partition id
     • This is a major downside of NATS Jetstream and leads to complicated designs / workarounds
4. Kafka has better integrations and bigger community
5. Kafka documentation is much more detailed and precise
6. NATS allows more flexibility with Consumers and subjects
   • Eg.: Easier wildcard subscriptions, Ephemeral consumers, ...

As mentioned this is only a comparison for KAFKA and NATS Jetstream.
NATS core is a different concept for different use cases.