Ultimately it's a question of the cooling efficiency of the PSU.
You can't really make judgements on the best solution without knowing how much cooling/airflow the PSU actually needs.
I'm not clue-y enough about the terms and numbers here to make a qualified statement, but here's what I'm thinking:
(All of these numbers are just made up, I'm not using real world figures)
If the PSU (with no fan at all) plateaus at 60c, but with the fan running plateaus at room temperature (or slightly above), then the question is; how much air flow is required to achieve this? I don't think you can just boost the fan to the point where the PSU ends up being below room temperature... So you only need to work out how much air flow is required to keep the PSU at room temperature.
If it takes 500 RPM from your fan to achieve this, but you can leave it at 800 RPM without hearing it, then you want it to be drawing roughly 300 RPM worth of heat from inside the case.
In other words, if the fan providing more cooling than the PSU requires while running at your preferred RPM, then you should also draw some
air from the case.
I'm not sure I'm explaining myself well, so look at it like a program:
IF FanRPM is Inaudible Then
IF PSU at RoomTemperature Then
Draw X air from Case
Do not draw air from case
Bugger. Buy a quieter PSU/fan.