The fact that the "hose" is usable infinitely in Atlantis regardless of what power characters do is huge. The trash can and the hydrant can be taken away, gadget's options in Atlantis never are.
That, and the pole in the middle (also infinitely usable) is helpful for a lot of gadget characters in certain matchups. Yes, power characters have better options, but so do you.
Yeah, but at the same time power characters can inflict much more damage on you with the interactibles in Atlantis, if they go for it.
It's easier to avoid the interactibles on Gotham Alley, since they're not full screen / tracking.
I do see what the advantages of Atlantis are, but at the same time these advantages are only better for a certain type of gadget characters. Everyone who wants to go in will care less about the corners on Atlantis and be more afraid of meter burned versions of the wave coming from power characters.
I personally would rather take the Gotham Alley, since even if the power characters destroy the interactibles, it's a neutral ground then and for me it was easier to avoid the interactibles.