If you like Raglan, Yoke, or Drop-shoulder sweaters, than EZ's system is very straight-forward, and you'll love it. (I've not read the book, but I'm familiar with the concept, from watching her video series. The system may work for other type sweaters too, but I imagine it would get complicated.)
If you like set-in sleeve sweaters, you'd probably prefer the Handy Book of Patterns, since the shaping for the armhole is irregular and specific.
The Handy Book is great to have, in any case, to refer to when you want to make gift mittens, hats, socks, etc. You'll always be able to find the size you need, and the gauge for your yarn. It's a great reference.
Also, with EZ's sytem, you need to be able to measure the person you're making it for, or have a sample sweater on hand. (Unless she includes sizing charts in the book? Not sure....) With the Handy Book, the sizing is very straight forward. Although the smallest size is 2 year old, and goes up in 2-year increments. Might not work for a small newborn, but for the most part, rounding up a child's sweater by a year or two is a good idea anyway.
Hope that helps!