Thank you WR3ND for being the first person to actually answer my question. From that link I can see that a Diamondback is in fact the next combat ship for me to buy and costs just under 565,000 CR. IF I were to go with the Empire my next ship would be the Courier and then Vulture if I decided I was happy with just two large hardpoints.
Weapon loadout on the DB and the Cobra is excactly same. DB has more utility points and is slightly more maneuverable. You might feel it is a poor upgrade. Just a heads up. The only advantage with the DB is the positioning of the hardpoints, which gives you more accuracy with fixed weapons.
Now, the imperial Courier, on the other hand, is a step up. It has 3 medium hardpoints, 3 times as strong shields, room enough for extra fuel tank and shield cell banks, with more to spare. Actually, the couriers shields, with a 3A shield generator, are more powerful than the Clipper's shields when equipped with the 7A generator.. imagine that.
A fully upgraded courier with 2 gimballed beams and 1 gimballed cannon - and it has the juice to use this setup - will cost you just shy of 8M cr.