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.
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....
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 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?"
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.