Scyther is hands down the better Pokemon and choice, stat and movepool wise. The point of a Nuzlocke is to use whatever Pokemon you get from the luck of the draw, this way your forced to use lesser Pokemon sometimes, to see if you can make it through. It's also an incentive to use a Pokemon you normally wouldn't in a playthrough. I say stick with your Ariados for the sake of the Nuzlocke, but Scyther is the better choice.