They're totally different players imo. Wide left I'd have to say Gervinho probably but if it were a case of it doesnt matter what position they play we just have to choose one to play I'd say Arshavin. He's a far more talented player. On his day one of the best in the world. Look at Rosicky, same age and has got it back. Arshavin can too.
Gervinho isnt a clever footballer, he's a tremendous athlete, great dribbler and that is incredibly useful for a winger.
but Arshavin is a finisher, a chance creator and has vision Gervinho can only dream of. I don't think I'll ever see gervinho score four goals in a game let alone of the quality Arshavin managed. Or the goal he scored against united.
Ideally I'd play Andre in the middle and gervinho out wide interchanged with ox and theo but that wasn't the question