Mass Drivers: Mass or rate of fire?

Railguns have higher velocity; coilguns can shoot larger masses more easily.

Usually railguns are preferred, and these usually take the form of 1g sandblasters. This is because minimising time to target is the single most decisive factor in a dumb weapon's effectiveness in space. Between a pair of closing gunships, all other things being equal the first to cross the line where its engines no longer put out enough thrust to evade the enemy's fire has lost 9/10 times. However this does assume the ships would actually be trying to close with each other. Lasers/dones/missiles/etc. all complicate this assumption and so the question can become "what makes good CIWS" rather than "what makes good antiship gun" when nobody wants to get within a Gm of you before throwing ten thousand missiles your way.

Anyway, I'd say ~300ms reload and 50g is good for a heavy gun. Low rates of fire (>1s) usually result in underwhelming performance owing to the low chance to not only hit, but hit something vital and damage it. Given the prevalence of whipple shields, deep aerogel layers, and the effectivenss of extreme armour sloping, your projectiles either need to be very massive (meaning low velocity and/or ROF, so undesirable) or you simply need to dump as much energy into the target as quickly as possible (meaning high velocity sandblaster giving you range, ROF, and rapid albeit inefficient energy transfer). If you've got a gun that shoots homing projectiles that changes things a bit, but as far as basic tracer shooters go the sandblaster is the primary weapon. Note what you said about the sandblaster - good at destroying modules — once you destroy all the external modules the enemy is a non-threat, armour or not. Heavier guns are still useful for various purposes like sniping particular modules off with high accuracy at the start of an engagement, but I'd say density of fire is important to cover first before additions. Consider superguns to be your secondary/special/utility weapons.

By the way, high velocity in the context of many discussions here is >30kms. Just so you know where your designs might end up. :P

Railguns

Pros - 

• Can achieve very high muzzle velocities (upwards of 115 km/s in some larger guns)
• Encounters fewer overheating issues compared to coilguns per similar muzzle velocity 
• Doesn't require a magnetic projectile, which opens up a wider variety of armature options.

I wonder how realistic those tens or or hundreds km/s railguns actually are - rail/armaruee contact at those velocity isn't going to be pretty.
That said even stock sandblasters (60 and 100MW turreted coilguns, 6mm railgun) are pretty damn effective at stripping everything outside of armour and, after tweaking for better accuracy, also become quite effective at drilling through.

When in doubt, you can always mix and match - given that during an intercept you are not going to keep your distance and that if there is any significant perpendicular component to your relative velocity, projectiles travelling at different velocities will impact from different angles (defeating sloping) mixed batter generally works better, especially in deep gravity wells.

Deep gravity wells also tremendously increase effectiveness of massive projectiles - if you are intercepting retrograde at some 30km/s relative (say, low Neptune orbit), it doesn't really matter if your gun adds 2 or 8km/s to your projectiles but whether your projectiles are 1g or 1kg will change a lot.

Overall, I like playing with stock components, if only because beating stock ships with hyperoptimized weapons feels like beating up a toddler.

Something seems wrong with the gun on the left. Either it could get a much higher rate of fire by increasing loader power, or that's a capacitor coilgun. Capacitor coilguns (at least in my experience) are far less efficient and effective than capacitor railguns with similar stats. I find coilguns are better when you want to shoot a normal slug (instead of a payload) at speeds attainable without capacitors. Once capacitors or payloads are involved railguns seem better.

As for the sandblaster, that could get through armor a lot better if it had better accuracy. Tiny projectiles seldom get through any reasonable armor in one shot, so you need a bunch of projectiles to hit the same spot. That's very likely to happen with something like 0.001° to 0.0001° deviation, but not with 0.01°. Any lower than 0.0001° and you start getting so accurate that it can struggle to take out radiators, just shooting straight through the ship at the base but missing the radiator itself.

Beryllium and zirconium copper are stock. Some of the modded things to check out are aluminium beryllium and an aluminium boron fibre composite.

So you don't beat up other players' designs from the workshop instead? It's a good way to pit one's design ethos against other kinds, too.

The left one is wrong since I used the wrong screenshot:


Also I've optimized my sandblaster, and made a heavy version of it.

I think I've done it! The super sandblaster in particular is a rousing success. It's faster, more massive projectile and now improved accuracy make it excellent at drilling though the enemy armor.