If you have the Unofficial Guide, and know it well, TGM can provide some additional tips that make it worth going through.
If you have not read the UG a number of times, and understood/digested the information that is in it, I would spend my time with the UG.
The UG is easier to navigate, and you can take it with you. I know the UG inside and out, and don't typically look in the index anymore, as I know where things are.
I have both, and like TGM, but I think TGM a better resource for someone that already has a good idea of what they want to do, and how to do it.
Great additional info on things like parade viewing, touring routes, and when to do things.