Start with the Lightning 28" you already have.
Personally, I don't shoot much differently with a really long barrel. 30-32" can be nice, but it's far from necessary, especially since the 12 Gauge Lightning is an overweight gun (good for trap). I wouldn't use a light 26" gun, though.
You'll know if you want a trap-specialty gun, and what you want, AFTER you shoot some, not before.
I was never particularly good with, and certainly not better, with my 34" trap single. I do fine with a 28" 12 Gauge Beretta field O/U, and I can use that gun for something besides American Trap. I sold my 34" right about the time I got a dog and needed a 12 Gauge break-action for gunning at hunt tests. Haven't missed it.
I do some after-work local league shooting, not ATA, though, and have no immediate plans to do that.
Also, if you are a bird hunter and you want to shoot more interesting and field-applicable, but unofficial, rounds, like Wobble, or even better, Progressive Wobble, a specialty trap gun isn't the best choice anyway.
Actual registered American Trap involves very limited gun movement, compared to other shotgun disciplines (unless you are shooting in 20+ MPH winds). Hence, real Trap guns are designed for smooth swings in a limited range of horizontal motion and almost no vertical movement. Some shoot extremely high, so you "float the bird" way over the rib in your field of vision. They can range from suboptimal to useless for anything else, even Wobble Trap, and they can also detract from your overall shotgunning skills. But ATA is serious competition, so it uses specialized guns just like benchrest rifle, etc.
Hence, I'd start with the Lightning, and go from there. That's a pretty good middle-of-the-road shotgun, not too quick but not too slow, and its heft is helpful at the range.
