I'm very much in favor of simply "Colonists" "Citizen" implies I'm part of something centralized, and if later down the road we're getting multiple star systems, I might just want to plop down on a lonely asteroid and set up my Hermit mansion in the far reaches of some distant solar system. Arker and its variations is too forced, and doesn't really convey any addition value to my role play experience. If there were people who DIDN'T come from the Ark it might be relevant, but I think as it stands, since we're all from the Ark, it's a meaningless distinction.
The only other suggestions I like are "Survivor," and to a much lesser extent, "Pioneer," because they convey the heart of the Role-play experience with out the forced and universe specific overtones of the Ark.