Which is preferred in this case?

I want a field to display an image. Each time we go to a different record, a calculation is performed to determine which image to retrieve from a global container field.

I can create my display field as (1) a Container field with the "Auto-Enter" option "Calculated value", or (2) a Calculation field whose result is a Container.

Which works better? Why?

It all depends on the purpose for which you'd like to achieve. Please note that "auto-enter" calc will NOT self evaluate, it must be triggered in some way unlike "a calculation" field that is self triggered. Merely moving from record to record will not trigger an "auto-enter" calculation to re-evaluate. You probably need to give more info as to your ultimate goal. both have advantages and disadvantages, it all depends on how much you can compromise.

My ultimate goal was a field that updated when the current record changed. That means a calculation field, unstored. Thanks.

If the trigger field in your record is of the same table occurrence as your container, you can accomplish what you want to do with an auto-enter calc or an unstored calculation. As Breezer said, it all depends on what you want to achieve. And no sense in storing a calc if you don't have to.