Difference between Dual Heavy and Dual Cannons is this:
The base DPS over time for both the cannons is identical.
The difference comes from the burst vs. continuous damage. Duals shoot faster but have less chance of inflicting crits. Heavies fire slower but have a 10% innate ability to score crit hits. Basically a Dual Cannon is a very good tool to drag down shields and provide a continious heavy damage output source while the Heavies are better to pound the hull to pieces. That's why a lot of people go with a mixed setting on those. I'd recommend personally that you try a Dual, Dual Heavy and your DBB if you want in combination with a good torpedo. Usually Quantum torpedoes are used on escorts. I still think that Photons are better used on Cruisers and maybe Sci vessels.
Not really, dual cannons are good for sustained damaged, while heavy dual cannons are better for burst damage since their individual hits hit harder then those of a dual cannon. One could also argue that heavies have just half the change of duals to trigger the weapons proc, since those are triggered by the amount of shots, and heavies have half the rate of fire then the normal duals.
But unless you are using phasers its not worth thinking about until the procs are reworked.