Yeah, personally, I think the Prom-S is the best overall gun, especially if you're better at gunnery than flying(or, at least, maintaining a tracking shot on a target). Unless you can really use the Kayser's fast reload time to consistently drive the damage, the Prom-S does 20% more damage(per-shot) to shields, while the Kayser uses 20% more energy, and gains one whole extra point of damage vs hulls(all also given per-shot).
You also get increased range(longer than nearly all non-beam turret guns), +100m/s shot velocity(arguably increasing accuracy due to needing less leading of the target, and also in compensating for changes in direction), and even if you are already accurate, you still get to spend that saved energy on shields/gun -> shield energy transfer, or engine/afterburner recharge, as needed/useful.
Not to say the Kayser is bad, but more Awesome but Impractical, to the Prom-S's Boring(or at least, 'less exciting') but Practical.