I think that would depend on your background and your knowledge of Bio.
I mean to say if you have recently taken bio classes and all the topics covered in Dat are still fresh then KBB is good enough. However, if you have taken Bio, long time ago, like in my case, then I would strongly recommend using Cliffs, till today I have just used KBB and am getting killed in all my practice tests - mainly because most of the questions - not covered in detailed in KBB look completely new to me.. I was actually scheduled to take the DAT on 15th of this month but considering my practice test scores i rescheduled it for 21st. I am definitely reading the Cliffs AP.
Bottom line is it all depends on who much u remember from you Bio classes in school and how you do on your practice tests.
I hope this helps.