Well, you do get diminishing returns from research, and each weapon has its own good uses.
I suggest dividing your weapons research according to how much you use each weapon in practice, with a modifier for how you plan to use them in the future.
Plasma and Lasers are good offensive weapons, since you can keep fighting deep in enemy space without needing reloads.
DUC and Torpedoes are good defensive weapons, since they have more firepower/range and if you're sitting at home, you can rearm easily.
Missiles, well, they're even more so. Use missiles in overwhelming volume with spreadfire strategies to saturate the defenses of a large enemy fleet. Then send the ships home to drain your homeworld's stockpiles dry.
Given the horrifying rate at which missile and, to a lesser degree, torpedo ships drain ammo, I like to build them on smaller hulls with a 40mm Plasma Cannon.
That way, they can unload on the enemy from max range, and will then do a suicide run on any remaining enemy forces.
They absorb fire that would otherwise go towards ships of the line, and they dispose of themselves so that I don't have to send them home for reloading. (With the small hulls, I can just build more and this saves on maintenance too)
I suppose it also saves me the trouble of doing retrofits, to boot! All the new ships end up with the latest tech; cannon fodder tends to be more advanced than the dreadnoughts.
Three or four birds with that stone.
