We go to WDW annually and buy both TGM and UOG. They are really quite different. Other than the format difference, the thing that strikes me is that the unofficial guide has page after page of ratings of all sorts of things, without any particular substantiation of them. The web site guy, on the other hand, fully explains the reasons for all of his recommendations. I like that. Their methods of handling touring plans are similar.
Also, I have learned more new things from TGM, at least the kinds of things I thought were helpful.
We got a friend a TGM subscription for her once-in-a-lifetime WDW family trip. She is ecstatic about it! She spends all of her free time planning itineraries and working the forums.
All in all, Tour Guide Mike is much more useful for me. IMHO, of course.