I think Green Arrow would be easier to pull off on film in a team setting. With Aquaman most people will ask why he is on the team if they're not fighting ocean based threats. If they are fighting ocean based threats then the casual audience might feel like it's too convenient to happen to have a villain that needs Aquaman's assistance. Sure Green Arrow being a rich guy with no powers may come across as just another Batman, but that can open up a rivalry there. You can have your cake and eat it too though. You can have one of the powered members of the JLA ask Aquaman for a favor. An example would be Superman is trying to find out what the villain is doing smuggling nuclear arms through some docks. So he goes to Aquaman to ask if he's seen anything, and we can get to see a little bit of Aquaman. If the fans like it then more Aquaman in the future. If they hate it then at least you didn't waste too much time on him.