Pretty broad question, as there is no hard and fast answer.
It really depends on the ship and the size of the PA.
Efficient is always a good first choice though, as it still gives some Damage/DPS boost, while reducing the heat produced substantially.
However.. if used in conjunction with the Thermal Conduit effect, then heat is your friend, as it increases the damage output.
Sturdy is also a viable option if heat control is an issue, as this will also reduce the heat produced.
Overcharged can deliver a significant damage increase, far more than Efficient, however it also diminishes the amount of ammo for these (negated by Plasma Slug, but means more frequent reloading), as well as greatly increases heat production and power draw. The biggest over-powered gun in the galaxy does no good if you can't power it!
Ultimately it really comes down to how you plan on using them.
Personally, I like to use mine like this:
[video=youtube;Eb7dYMY96gw]https://www.youtube.com/watch?v=Eb7dYMY96gw[/video]