You will find that if you wish to change weapons after buying them, they will stay with the ship you bought them for, but they don't show up on the right menu, so you can't sell them if you wanted to. They are a great help. You might want to rapid-fire after the ship turns around and flys away from you, thats when they seem to fly straight.
I have found that Tacyons seem to be the best buy, it only takes 2-3 shots to destroy a Talon (with 4 on a Demon mind you), but I recently tried 4 lasers (they fire at .30 Tacs at .40 ie faster) and I seem to kick but just as good as the "heavy" gun. So, if you need money to buy something else try lasers w/auto-tracking for a while until you can get the more expensive ones.
Have fun!
Starblazer