There's a difference of roughly 600 damage per minute PER cannon.
All things being equal, your lower damage per shot on a DC vs a DHC, but both having the same chance to crit, would work out to the same DPS regardless of crits. Once you throw in a + Crit Severity weapon (assuming it adds static damage) the Dual Cannons again out pace the Dual Heavy Cannons, by adding that staitc damage more frequently.
Add to this the fact that they cost less power and you'll see why I just bought all Phaser Dual Cannons for after the respec.
Also, more shots per minute means more chances to proc the sub-system disabling abilities of a phaser. I'd be getting 10 more chances per minute off of each cannon I slot. A dropped shield is much better than a small DoT or resistance drop. This would allow me to avoid fire for a few seconds or sneak in a HYT3 when shields go down.
meh i tend to go for those feds that assume they can roam about alone as if they are in some pve zone. Atm they die in a few seconds, my vm sits at 32 seconds, regardless of what aux is, so adding a couple more seconds to the kill time really wont, for those poor isolated souls, make any differnce.