In this case, what way should the PSU go in?

Depends, I suppose, on what the case is going to rest on. If a deep pile carpet, then get intake from the case. If smooth lino, then from the bottom. That looks like a good dust filter over the vent, but clogging may also be a concern down the road.

With mounting holes that allow for psu installation in either direction, what the manual states is pretty much useless.

I have an Antec 900 case. I suppose that I could cut a hole in the bottom of the case, but I install the PSU with the fan up. The lower fan in front blows cool air over the hard drives and the PSU exhausts the air. Air temp outside the PSU has never been more than 4 C. above ambient.

In my Antec 1200, I installed the PSU with the fan pointing up. Simple reason was to create more negative pressure in that area of the case forcing greater airflow towards my GPU's intakes.

While pointing it towards the bottom makes sense from a ventilation perspective and minimising unnecessary heat in the PSU, dust would be a major concern of mine. Last thing you want is your PSU getting clogged up, overheating, burning something out, then going up in a fine light show taking half your system with it  

The manual did not state anything.
So I called Thermaltake today, and they said that either mounting position worked...it just depended on the amount of fans/abient temp/setup. In my case, they suggested I mounted the PSU with the fan up.

Thanks for the help.