When should I use Redshift non-Serverless over Redshift Serverless?

In general, below two principles can be applied to any serverless vs provisioned deployment

Go for serverless, when

1. Applications that do not expect steady state and constant traffic, for Example, dev or test or adhoc project
2. Application that expects steep spike or unpredictable demand

If application expects to be steady state with predictable demand then go for Redshift Cluster with RI's

As per Redshift Serverless documentation:

Amazon Redshift Serverless is ideal when it is difficult to predict compute needs such as variable workloads, periodic workloads with idle time, and steady-state workloads with spikes. This approach is also a good fit for ad-hoc analytics needs that need to get started quickly and for test and development environments.

Provisioned Redshift on the other hand may be suited for steady predictable loads.