Well, for me, early on, I tended to use a mix of the fast and slow fire rate guns.
But something I noticed once I reached Kusari space and bought a Baracuda may be of interest.
As I started otufitting my ship with the 2 lvl 5 guns and 4 lvl 6 guns, I had many choices. You see, why flying thru Bretonia I had been getting loot of lvl 6 guns so I had been saving them all for my next ship upgrade since I wasn't sure which would be best.
As I was outfitting my ship, and trying to figure which combination of fire rate/energy usage/damage would be most efficient, I started paying attention to which "shield" types each gun was best at. I noticed several different guns worked best on the same shield, and worst on the same. I also noticed I had other guns that were the opposite.
So, I decided to use 2 of each type. I ended up with 2 guns with say Molecular/graviton, then 2 with Graviton/Particle, and 2 with Particle/Molecular as their strengths/weaknesses. And when I went out the and starting fighting, I found I had no problems with any combatant. And considering you can't control who you will be fighting in every situation, I think you will find it best to not pay so much attention to the damage and rate of fire, but be sure you don't have all your guns of one strength/weakness, unless you KNOW you will only run into combatants with the right shield type.
I now have a Dromedary freighter, with it's 275 cargo woot!, and have done the same type of thing with it's guns and turrets. So regardless who I run into, I have at least 2 guns that are best against their shields
regards,
RogueOne