P-Trans > Gunboat, I think. Pity many people don't think that way. Not only can you blow up a trader that refuses to pay, but you can suck up all his cargo and take it back to base for profit. It also has enough fire power to take out a Gunboat, and luckily since most Gunboats in Liberty mount 1-2 missiles, you have an extra advantage.
If it were up to me, the Outcast Guard ID would not be allowed to enter Liberty at all, the Liberty Rogue Guard ID would only be able to operate in Independent systems (Gallieo, Kelper, Bering, Hudson, Magellan, Cortez, Ontario(?), Alberta, Yukon, Cassini etc.) That would stop those Destroyers from messing up the place. Similar restrictions should be in place for the Lane Hacker Battleship too, I think. It's supposed to be a very rare prototype ship...