Obviously I know which one I'd pick

Seriously though, I'd suggest checking reviews etc. and see what other readers think. There are many referenced for my book on the Ranch's own
SCWCD Links.
Amazon.com and
Amazon.co.uk are also useful places to see what reviews have been written. There is also an interesting comparison of the two books you mention on
Amazon.de (you'll probably need a translator like Babelfish!) Then I usually check the publisher sites too for detailed tables of contents and sample chapter(s), and see which style best suits (Head First is beginner-oriented and very 'jokey' whereas mine is more of a straightforward textbook-style study guide).
Of course, there is no reason not to buy more than one resource if you can afford it - different perspectives are always good...
Charles Lyons (SCJP 1.4, April 2003; SCJP 5, Dec 2006; SCWCD 1.4b, April 2004)
Author of OCEJWCD Study Companion for Oracle Exam 1Z0-899 (ISBN 0955160340 / Amazon Amazon UK )