well, the easiest way to rate defence potential is rating it in hit potential - essentially, by tracking speed of missiles destroyed per shot.
so for these fighters, the first one fires 4 shots at 13,278 km/s with a fire control accuracy of 86% this gives it 4*13,278*0.86 = 45,676 km/s of incoming missiles destroyed per salvo - against AMMs traveling at 24,000 km/s that's about two missiles destroyed per fighter. The second fighter fires 4 shots at 16,427 km/s with an accuracy of 69%, for 4*16,000*0.69 = 44,160 km/s of incoming missiles destroyed per salvo. The two designs are extremely similar in their performance with a slight advantage to the 'base' version. Since speed is an advantage itself - especially for beam warships - I would recommend the second design.
I would prioritize getting your railgun velocity up to 3, though. As it stands, a ship traveling at as little as 4500 km/s is immune to your fighters if it has the initiative advantage. (It can travel 22,500 km in a 5 second increment, enabling it to always stay outside your fighters 20,000 km maximum range.) Though, to be fair, the second designs less-capable fire control is incapable of effectively combatting anything at that range...
I would also recommend removing the maintenance section and replacing it with a minimum-sized active sensor. You probably won't be able to fit a sensor that can detect missiles, but an active sensor would enable your fighters to operate independently and remain combat-viable if their supporting vessels are destroyed or crippled.