This is more of a toss-up or opinion in terms of how you want to design your whole architecture. Here are some things to consider:

Same cluster:

• Pros
  • Workloads don't need to go to a different podCidr to get its data.
  • You can optimize your resources in the same set of servers.
    • This is one of the main reasons people use containers orchestrators and containers.
    • It allows you to run multiple different types of workloads on the same set of resources.
• Cons
  • If you have an issue with your cluster running Cassandra you risk losing your data. Or temporarily lose data if you have backups. (Longer downtime)
  • If you'd like to super isolate the db and app in terms of security, it may be harder.

Different clusters:

• Pros

  • 'Safer' if one of your clusters goes down.
  • More separation in terms of security for your data at rest.

• Cons

  • Resources may not be optimally utilized. Leaving some CPUs, memory, etc idle.
  • More infrastructure management.

Different node pools:

• Pros
  • Separation of data at rest
  • Still going through the same PodCidr.
• Cons
  • More management of different node pools.
  • Resources may not be optimally utilized.