A little late, but as I fly gatflak junker regularly in pubs (and well, at that), I felt obliged to put some thoughts down:
For pubs, I prefer mirrored gatflack with the front nose gun as a flare or some other utility gun. When I've got a good crew, I'll usually switch one side to a longer range focus, or replace a gat with a Hades and swap the other side to a brawly/disabl-y carronad-banshee. Whatever I feel like, really.
I don't find the arming time problematic on the flak gun with the mirrored build in casual play. I can usually stay at arm's length away, on the edge of the gats range. Plus, i find that the flak punishes the enemy for mis-maneuvering into my arcs more. Also, the gat and flak are easier to aim for random pubbies. (But even then, I get a player that can't lead worth a darn with the flak, let alone mortar.)
I guess it's because I play my junker like an escort rather than a brawler or sniper. I just stay near my ally and cover their sides from medium to close range. It's worked well for me so far. I prefer the flak because the playstyle is passive, and I find it harder to cover my allies with the short range of a mortar or odd arc and the arming time of the hadies. The gat-flak is that "sweet spot" off of 120 meters where the enemy ships are most of the time.
It's also easier to coordinate by saying: Shoot the gatling all the time, and then wait until the red to shoot the flak. That way, when I change sides, spin to avoid a ram, or do some other evasive maneuver everyone knows that their job is the same, just on the other side. I love it when a pyramidion overshoots and my gunner instantly gets on the opposite side to continue shooting as if nothing has happened. Also, the flak is much easier to aim in the thick of things while spinning and dodging enemy fire.
So yeah, as of now, I think the range and accuracy of the flak is better suited to how I play "escort" junker than the close-in range of the mortars.
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Of course, when I get a good organized crew (like many people coordinate to do), I'll change it up to play with what my crew is good at/wants. And make it something that requires more knowledge and coordination.