I've read both books so I'll take a stab at this one. First, don't underestimate the value of being up to date. There have been some significant changes in Groovy since GinA was published. Last I heard there are no plans for a second edition of GinA until Groovy 2.0.
Next, they are different books with different focus. For example, GinA has more thorough coverage of Builders whereas Venkat's book (PinG) has much more thorough coverage of metaprogramming. In fact the MOP stuff alone is probably worth the price of PinG.
GinA may also server better as a reference while PinG would be a better choice for a
Java programmer's first Groovy book. It's shorter and a bit easier to read.
Another point is Venkat's background as an instructor (professor, trainer). This really shows in the way he explains the tricky topics. I have been working with Groovy / Grails for over a year now but I still had several "aha!" moments while reading PinG.
Bottom line, I heartily recommend Programming in Groovy even if you already have GinA.
Originally posted by Venkat Subramaniam:
I am going to wait for someone who's read both books to answer this question