The Andrew Monkhouse book is pretty specific to SCJD...and does map closely to what you will need to do for this certification.
Whereas the Head First Patterns book covers OO design in general, through the medium of Patterns and it really, really, makes you think. It's well written with good examples (actually pretty funny in places too).
I reckon it is a surefire certainty that you would be a better designer for reading the patterns book.
SCJP 1.4 (93%)<br />SCJD (In progress. It can run, but it can't hide...)