Depends somewhat on how quckly your jurisdiction adopts the new code. Some adopt it effective January 1, and some may delay a year or more. At any rate the 2008 is comming sooner or later, so why not buy the most up-to-date version?
Why not the 2005 now and the 2008 in a year or two? Even if 08 is never adopted in your area, you will have it for your collection and future Forum discussions.