Don't know about the Williams but the Enzo is a state of the art model...go buy it!!! In fact get as many as you can afford! It's a truly amazing kit that you build in the same manners the real thing is assembled. Many parts but in no sense difficult to build. The only thing that needs extra care is the body, as it's made up from a lot of separate pieces that needs to be carefully aligned.
From what I've seen the Williams is a more than descent kit with some new features like separate brake disks and calipers where the disks rotate with the wheel like the real thing. Get both of them, 40% off gives a very good prize.
Take your time building them, or better yet...save them for a while and try something a little easier if you find them somewhat intimidating.
That's my
Forza Ferrari!
Niclas