I have the fan facing inwards. Mainly because I find it easier to manage the 24pin that way. However, I also run a side radiator with the fans intaking pretty directly into the PSU.
My thoughts on it are that the PSU facing inwards will pose less resistance to the airflow than if it was blowing against the flat back on the PSU. I'm planning to swap it around soon and have the radiator fans exhausting out the side with the fan on the PSU facing outwards though, just to try different things.