I think it depends upon the amount of flow resistance in the radiator that you are trying to overcome.
With a dense radiator, you might have to work very hard (i.e. high RPM) to get any airflow, and two lower RPM fans might be able to get lower noise for a given CFM.
Personally, I prefer a wide radiator with room for two fans pushing air into it (using a 6" gap held by ducting). But, that takes a lot of space.
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