GA : 260 visitors for one day (30/04/2010) Urchin 6 : 750 visitors (30/04/2010)
Then, who believe ?
Urchin analyzes your logs and uses heuristics (in layman's terms, guesses) to try to extract concepts like page views, visits and visitors. It identifies some webrobots, but fails in filtering them all out.
So, while reality is probably between those numbers, my experience tells me that Analytics is closer to the truth.
Compare what GA and Urchin report against your webserver access logs. Then you'll have some idea of how much traffic actually hit your server.