So I concur with what has been said above- It all depends on your setting and what you want to achieve. Gatsby is a great book test, especially for the price- Zodiac is a killer because of the method but is tough if you are just walking around.
In terms of practice, at their core both are pretty manageable, it's just a matter of what your routine looks like. I'm a huge fan of presenting someone with a bunch of books, if they pick Gatsby, I go into that method, then I put it aside and move into Zodiac.
Both are killer- can't go wrong with either.