I'm another who says both. Last year I used the book and the website for UG--it's confusing but while you can get the crowd calendar if you own the book, you can't use the UG planning tools, which are actually quite helpful, if not very high tech.
For this year's trip, I signed up for TGM, and was initially quite disappointed. I found it very disorganized and confusing, it logs you out and makes you start back at the home page to log back in, it has no planning tools, and I have encountered some really egregiously out-of-date information (Tusker House review without even an indication of a change, much less an actual new review, Tomorrowland CS dining, particularly the latter being outdated to the tune of "no re-opening date as of summer 2006"). There really isn't anywhere on the site to manage your ADRs and write out your plan, which the UG does have (for $8 whether you have the book or not).
Given all this, I was a little upset at the TGM cost at first, but then I re-upped on the UG site and I saw where the value of TGM was. First of all, the two sites' crowd calendars are really different (actually contradictory during our time frame, April 18-28). I respect UG's methodology, but despite being a data analyst myself (or perhaps because of it), I like the discussion of the judgment calls TGM makes in deciding which days to recommend. It helped me decide whether the recommendations made sense for me. I also like TGM's discussions of touring strategies and tips that are more general, and therefore more flexible, than the UG's plans. On the other hand, the UG book itself has much the same sort of tips, and they tend to be more honestly critical. I found TGM's reviews (of dining particularly) to be a bit too colored by mouse-shaped rose-colored glasses.
So as another poster said, get 'em all: the UG book, TGM, and the crowd calendar. It's a drop in the bucket, in the end, moneywise, and all that planning makes a huge difference--my partner keeps telling me that all the planning I did made our trip great last year.
Cate