I'm a satisfied customer of both UG and TGM. My take on both: if you're spending $2000, or more, on a Disney vacation, why not plunk down an extra 2% of the cost and buy them?
I just don't see why folks on both sides of this issue get so excited. Well, actually, I suppose I do. The DIS operates on a community vibe, and TGM's information isn't intended to be shared that way. (And, I suspect if you asked Len directly, he'd tell you that the UG's information that isn't already public isn't intended to be shared that way, either. Does anyone else recall the prohibition on posting snippets of UG's crowd calendar before it was published in toto at touringplans.com?) On the other side, the TGMers are quick to defend a service they really like---in part because many of us view TGM as a personal service rather than simply a reference.
In any event, if you are an infrequent visitor, and don't have a half-dozen trips under your belt, some outside resource is useful. Many people use and like the UG. Others use and like TGM. Some of us use and like both, incorporating from each as we see fit. When both of these resources have something to say on an issue, eight times out of ten they agree. For example, their touring plans are similar, in part because they are common sense: get there early, visit the attractions whose lines build fastest first, etc. The ninth time, they may not say the same thing, but it's probably not a crucial difference---after all, there isn't only one way to visit WDW and have a great time.
Maybe one time in ten they have a different opinion, and it can make a difference, but the difference often boils down to *how* you want to experience WDW, rather than one being "better" than the other. From my perspective, the two resources have a different philosophy. Both are fine approaches, but one has to decide which approach is best for them at any given time.
One difference can be seen in how the touring plans are constructed. The UG wants you to see every attraction you can possibly see, and the touring plans are designed to help you do that. After all, you've paid a lot of money to be here, so you should try to do things you can't do anywhere else. TGM wants you to focus on the best Disney has to offer for your party, and to take that at a pace that doesn't require you to take a vacation to recover from your vacation. So TGM touring plans typically have a list of "must do" attractions, and a list of "maybe-if-time" attractions, and the plan is designed to make sure you see the must-dos and can incorporate the others as you like---a TGM user tends to ask "should we stay and see one more attraction, or would my family be happier if we went to the hotel pool now?"
Another difference comes from Mike's background---he's a paid guide, and so he thinks like a host, not necessarily like a family member. If your family's planner doesn't mind occasionally taking one for the team, then Mike's plans have some interesting wrinkles that I've not seen in any other formal resource, and that are pretty neat.
A third difference is that UG has good offsite coverage---TGM doesn't really consider life outside of The World. In contrast, I've found TGM's park/resort coverage to be more in-depth.
Contrasting both UG and TGM to the DIS: for any question posted on DIS, you'll get ten responses, with six different answers. With the infrequent exception, there is never consensus on *anything* around here. That's fundamentally because. when it comes to matters of opinion, there isn't one right way to do most things. But, for an inexperienced visitor, that's more confusing than it is helpful. The UG and TGM each have slightly different perspectives, but they each speak with one voice, and that's often very useful to someone (like me) who doesn't have time to read DIS for an hour each day.
All that said: if you're someone who's owned an AP for a few years now, and you never bother picking up a guidemap, because you know three ways to get from Space to Splash without looking, then you probably don't need either UG or TGM. Likewise, if WDW planning is your hobby (hobby: anything on which one can spend unlimited amounts of time and money) then daily DIS purusals are probably more your style. That's fine! There's more than one way to skin a six-foot tall rubberhead mouse.
Just teasing. 
I used the same "wrinkles" at IOA and it worked splendidly... 
