A fixed end date is easier to manage. You know exactly when you are free to relist elsewhere or take the home off the market, without having to track a cancellation deadline buried in the contract.
If a brokerage insists on automatic renewal, ask for the exact notice period required to opt out and get it confirmed in writing, since a verbal understanding will not help you if a dispute comes up later.
A listing agreement is a business contract like any other, and reading the renewal terms before signing costs nothing compared to the position it puts you in later. Get your hand-reviewed valuation from Joel Dyck.