I wondered the same thing...is it worth it to get the new one? Being the planner I am, I knew I wouldn't be happy unless I did. I bought the Deluxe and I really like it over the spiral, so I'm happy.
That being said, there is little diff between the 2003 and 2004. The big diff is the maps are much better and in color. It has some little stickers you can put on your envelopes or in the book such as Happy Birthday, Anniversary, Food, Places, Arrows, etc. Plus it has info on Pop Century and Saratoga....but if you're not staying at these, use the park maps and don't care about little stickers, the 2003 will be just fine. Plus the info on PC and S have been updated since printing and you can download the latest info from the Passporter site.
The 2004 highlights the updated info in blue...from thumbing through the book it looks like 2-3% of the info has changed. Not much blue highlighting. If you're checking these boards often, you know all the changes already.
If you decide you have to have the new one....like I did

....it's cheaper to order it from Walmart.com even with the 30% off Passporter offers for ordering from them.
I know many others will have a diff opinion. this is just mine.