The best thing about TGM: you get one consistent opinion. Try using the DIS to decide whether or not you should go to an EMH evening in the Magic Kingdom. You'll get at least three different answers to a yes/no question! Which one should you listen to? It's never clear, especially for someone relatively new to the parks. With TGM, you get one answer---Mike's answer---plus his reasoning for why that's the answer. Since he makes his living as a paid guide in the parks, his answers tend to be well-informed more often than not.
The worst thing about TGM is, again, that there is only one opinion. If your style doesn't mesh with Mike's style, you can be on your own. For example, his touring plans assume you are willing to get to the parks at or before opening when necessary; there are only a few designed for later arrivals (and then only where it makes sense.)
My general advice about TGM is if you are a very experienced WDW visitor and/or have very firm ideas about what you want to do when when you get there, then TGM isn't as useful as it is if you are willing to try things his way. If you think it might be useful for you, sign up. If you don't like it, ask for a refund. My sense is more people are happy than not, but of course it won't be for everyone.
As for the OP's question of TGM vs. UG: I have (and rely on) both. They follow slightly different goals. UG's goal is to get you to see as much as possible, waiting in lines only when necessary---you can rest up after you get home. TGM's goal is to have you avoid lines, skipping marginal things that aren't really worth the time only when necessary---keeping you well-rested along the way. Our family prefers the more relaxed pace of TGM-style touring in most situations, but again, YMMV.
Either one will put you miles ahead of the average WDW guest, and the differences after that boil down to personal preference. If you're spending several thousand dollars on this trip (and you probably are) it's hard to argue that you shouldn't buy both.