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.