The difference is the trigger. A claims-made policy responds only if both the incident (after your retroactive date) and the claim happen while the policy is in force. Cancel it, and claims filed afterward aren’t covered unless you buy tail coverage.
An occurrence policy locks in coverage for anything that happened during that policy year. A patient could sue five years later and that old policy still responds.
Claims-made is cheaper in the first few years and is the more common form. Occurrence costs more up front but leaves nothing to buy when you leave. If you expect to change jobs or states, that matters.