I'm not sure about either system to be honest. The basic fact is that MS and Sony have designed Natal and Arc to appeal to the Wii crowd, rather than the more 'core' gamers the PS3 and 360 have managed to attract so far. However surely 'the wii crowd' by now have all already bought Wiis (or at least 50 million of them have) and aren't likely to go out and buy a 360 or PS3 as well. And the games that Natal and Arc will play won't appeal to the PS3 and 360 crowd for the same reasons the Wii doesn't appeal to the 360 and PS3 crowd.
Essentially, these devices are to go after a market segment that Nintendo have pretty much already saturated and, even if not, Nintendo have a massive head start in servicing. And they do nothing in the markets that Ps3/360 have a lead over the Wii in. From a business perspective, I really don't see how the Natal/Arc are going to add to their respective platforms.
Jon