I like the Cobra more, it's 'neater', because it doen't have bits sticking out, just one weapon, which does it's job, so you could say it's more 'Eldar', but that's another arguement.
Also, I think it's more useful than the Scorpion, as a 'guess' weapon which uses the large blast, and can move and fire, while the Scorpion has to see it's target, it get's it shots, but you get almost the same by fielding three Fire Prisms, but they cost less points. I'd say model-wise, they're nice, but probably not that worth it for points.