The book is going to be more detailed and will give you a number of things to think about. Snap is entertaining, but is pretty cut and dried.
Snap has some great bonus sections as well. The book has a.reed's pro spotlight from volume's issue # 2, which is not to be missed by any wakeskater.
So if you're looking for detailed instruction that may not be quite as entertaining (but remember, this is an instructional and not a freeride video), I'd recommend the book. If you want a few pointers on your trick and a lot of entertainment, get snap.