I'd say tracking speed, especially if you place them on a turret so they can track faster. Missiles can move very quickly, after all.
What I mean is WHAT tracking speed? What speed do you engineer the system to track at? 15k? 20k?
At what range do you engage incoming missiles? Do you use infrared lasers and engaged them close to the ship or use higher tech lasers to shoot at them further out?
Basically, how do you engineer a good laser PD system. I'm looking for tips because I'm working on one for my game.
While I hope someone will correct me if I'm wrong, the higher the tracking speed, the better. There's no way to tell how fast enemy missiles will be before they start shooting at you, so having a middle-of-the-road speed (say, 15,000kmps) should work until you get a better idea. Faster = more expensive, so if the enemy shoots big, lumbering missiles at you, go cheaper.
Farther range gives you more time to track and engage the missiles, which can be helpful considering how fast they generally move. A fire control only effective to around 10,000km won't do you much good if the missiles move faster than that.
For beam-based PD, I usually go with mesons. They go through armor and shielding to deal direct damage to the missile.
Here's some example technologies, using mid-range tech levels:
Fire Control:
Fire Control S16 96-16000
50% Accuracy at Range: 96,000 km Tracking Speed: 16000 km/s
Size: 16 HS HTK: 1 Cost: 230 Crew: 80
Chance of destruction by electronic damage: 100%
Materials Required: 57.5x Duranium 172.5x Uridium
Development Cost for Project: 2300RP
The actual cannon:
R6/C4 Meson Cannon
Max Range 60,000 km Rate of Fire: 5 seconds Focus Modifier: 3
Meson Cannon Size: 4 HS Meson Cannon HTK: 2
Power Requirement: 4 Power Recharge per 5 Secs: 4
Cost: 6 Crew: 40
Materials Required: 1.2x Duranium 1.2x Boronide 3.6x Corundium
Development Cost for Project: 1400RP
and finally, the turret:
Twin R6/C10 Meson Cannon Turret
Damage Output 1x2 Rate of Fire: 5 seconds Range Modifier: 6
Max Range 60,000 km Turret Size: 19 Armour: 0 Turret HTK: 4
Power Requirement: 8 Power Recharge per 5 Secs: 20
Cost: 67 Crew: 80
Maximum Tracking Speed: 16000km/s
Materials Required: 57.4x Duranium 2.4x Corbomite 7.2x Corundium
Development Cost for Project: 670RP
I hope that something here helps