They're basically identical. The website is updated a bit more frequently, but the updates are mainly minor wording fixes, and get incorporated whenever the book gets re-printed (every 6-12 months)

I'm fairly certain that they are identical. Hadley got the publisher to agree to allow him to keep the website up after publishing.

Well yes, but the interface to the site is much more annoying than the book (which is rather cheap for a textbook of this sort).

YMMV, but I got much more from the book than from the site, and was also able to give hadley some money in the process, so its win-win :)

Personally I prefer the website because I can normally remember which chapter something is in, and then I can find-in-page to quickly jump to what I was thinking about.

Aah yes the old "write a book to help one remember how to do it" trick. My technical output spans a book and various places on the web. The number of times I go googling for a solution, to find an interesting text ... then I wonder who wrote it, look at the author and discover it was me. It makes me laugh and cry at the same time :)