I have all 3 and like them all for various reasons:
1) UG is extremely thorough. They have what I call the 'cold hard facts'. It does remind me more of commando style touring - although there really is no pressure from the book, it;s just my opinion. I enjoyed their details, especially prior to my first trip
2) Passporter - LOVE it. I bought the deluxe edition and use it to keep all my tickets, confirmations, details together. We also use the pull-out park maps and find them VERY convenient, small clear and concise. The passporter also has alot more 'planning' type features - ie space to write down your budget, flight details, reports and ratings from readers etc etc. In addition, you can get updates to the Passporter online - but to be fair the UG has online updates as well
3) TGM - again, love it. The thing to keep in mind is that it is not a 'put in your itinerary and out comes detailed plans for each day' type of website. I value his best/worst days of the week for parks, and his forums which keep you UTD on things happening in your month. His reviews of restaurants, resorts, rides etc is very thorough. I like to use his site to plan my daily activities and record them in a .pdf planner he has available. There is a TON of info on the website and it does take a while to orient yourself and get through the info.
So, if you have lots of time, I would suggest all 3. If you only have a short time, I personally would drop the UG, but that's just me. The passporter is easier to take along (and the deluxe is 3 ring binder so you can take out the pages you don't need when you go to make it thinner) and TGM is good for pre-trip planning if you like to/have access to the computer/internet alot.
HTH
PS - Any other guidebook I have found is just too 'high-level' and everything in it I already knew from these boards. You will definately find things in the books you already know, but also alot of stuff you didn't!