PS3 Arc seems like an enhancement to the Wiimote, I think is will be easier to adapt to existing games and ports of Wii games. IMO, the Arc will be better for "hardcore games" than Natal.
Natal is something entirely new. It's technology will not work for many types of games, but I am more interested to see what developers come up with for Natal. It does still needs that one killer app to catch on, and I have not seen it yet. (Like WiiSports was for the Wii)