Beam weapons are good, but it is better to choose any different technology than the other races use. The reason: if most of the races use some weapon technology, they usually use the protection against this technology. However, if you have a weapon, that does not match their protection, you can beat much stronger foes: defender shield 16: gun 4 is as effective as laser 16 (4 = sqrt(16)).
In Dread Lords this was true but in ToA I've noticed that an AI species' weapon type seems to be constant from game to game - rather than all civs picking one of the three, each race has a type that their AI favors every time. For example, Drengin use mass drivers while Iconians use missiles.
A Thalan game I started tonight has the Korath using beams, Iconians with missiles, and Drengin with guns. I could have sworn Korath preferred guns like their cousins, so maybe there's some variance, but overall you can't count on an AI weapon standard.
On the other hand, the fact that ships with fleet modules are always targeted first allows you get more mileage out of defense: simply stack defense mods on a ship with any fleet module and it will tank for a fleet full of ships with nothing but guns.
