Cost isn't the most important thing if you were nuts about playing a lightsaber game with realistic motion control. If you already had a PS3, the choice becomes a lot simpler.
If I have two versions of the same game using comparable 1:1 controls, then I'm going to buy the one with the better resolution and graphics. There's no reason to buy a downscaled version unless you were really attached to the Wii.
But ultimately, it depends on what platform/control system the best sword action game comes out for. With the Arc and Natal, the Wii isn't the only option for motion control games anymore. Assuming either aren't using more dated, inferior motion control based systems (they aren't), there's no reason to believe that the controls for either would be inferior if the developers do the job right.