Subscription implies that your costs are roughly the same across different usage patterns by different users (otherwise you collect expensive users who under-pay for the resources they consume). Or, that you can tier your subscriptions in a way that makes sense for the bulk of your users.

Usage-based comes with more work in the short-term: more internal metering, controls for bill-shock, observability so users can see usage by billing vector, pricing/marketing work to help users understand their expected costs for different workloads.