I've got the official UKCAT one - I bought it off eBay. I just like the feel of a book rather than look a a computer screen. Really, if you learn better using the website then go for that, if not go for the book. Also, lots of other people on here use the book and have found it helpful and not alot have used the site I don't think so maybe I'd go for the book just because more people have used it than the website. However, the website might be excellent, I don't know. Just go for the one you're most comfortable with.
If you do go for the website, post a question or two up on here (if it's allowed) and then possibly TSR can see if they are up to the standard of the UKCAT to make sure it's giving you good practice.
Also, try medicmix.com