The more up to date versions have lots of useful updated info like liquid yeast info, updates for the better variety of ingredients available nowwdays, etc.
For the price, I'd buy the old one and read it from more of a spiritual/emotional perspective and supplement that reading with www.howtobrew.com
, which would be your source for up-to-date technical info. Papazian is a must-read, and I often re-visit each of the three versions I have (!), but I think Palmer is a bit more concise and technical...good for instant reference. PapaZ will educate you about the soul and spirit of homebrewing, and it comes across in all versions of the book.
That being said, if the $ doesn't bother you the more recent versions are better resounces, you've just got to be able to tell when you are reading an 'outdated' technique.