I buy the current Birnbaum Guide every year - so I'd say get both. If that's not an option, buy the 2004 Unofficial Guide this year and read it cover to cover, then get the 2005 Birnbaum Guide next fall.
I'd always buy the most recent guide. The only exception would be if you were going during a year long celebration that is highlighted in a certain year's guide and you'd like that info. Otherwise, buy the most current guide.