Which is better in JMeter: migrating to JSR223 or keeping BeanShell?

For any script that runs on every iteration of a meaningful thread/iteration count, which describes most scripts actually doing real work in a load test, migrating to JSR223 with Groovy is generally the better choice, since the cached-compilation performance advantage compounds directly with scale, and current JMeter guidance explicitly recommends this path for new and actively-maintained scripting needs.

Keeping BeanShell can remain reasonable for scripts that run rarely, like a one-time setup script in a setUp Thread Group, where the performance difference is negligible in absolute terms, or for an existing, stable, working script where the migration effort and re-testing risk outweighs a performance gain that may not even be noticeable at that script's actual call frequency.

A practical decision rule is to prioritize migration by impact: profile or estimate roughly how much cumulative overhead each BeanShell element contributes across the full test run, and migrate the highest-impact ones first, rather than treating migrate-everything or change-nothing as the only two options.

It's also worth noting migration isn't purely mechanical - JSR223 with Groovy has some syntax and API differences from BeanShell's Java-like interpretation, so a migrated script needs actual testing to confirm it behaves identically, not just a syntax find-and-replace assumed to be safe without verification.