The difference is simple. Natal is being taken very seriously by Microsoft, and the Playstation Eye is not by Sony. At the end of the day, that's how most things that become big in gaming do. It's not about being first, it's about doing something at the right time and shouting in loud enough. By the time Natal launches, it might have over 3 times as many games as the PSEye has, and that's after the PSEye having 3 years on the market. How many retail games are there again after two years? Eye of Judgment and Eyepet? Whoopdie doo.
The technology doesn't matter. The dedication to the hardware is the difference maker.