I have both Sehlinger's unofficial guide and Birnbaum's general guide (but not the one for kids). I am 21 and will be going to Disney with just my partner of the same age.
For us it has to be Bob Sehlinger hands down for the simple reason that it is a consumer geared product, which is to say it is primarily a critique of Disney rather than something to wet your apetite and make you smile (although it will do that too!). There is just so much information in the UG on the parks (including Universal), hotels, reastuarants, planning, shopping and a really interesting section on World Disney with kids. Be warned - it is a brutally honest book and will not sugar coat the pitfalls of Disney -an approach I like.
I also am incredibly impressed by the enormous team the unoffical guide has - its not just Bob Sehlinger, but restaurant critics, a local shopaholic resident, child pyschologists, a golfer, a computer programmer (Len Testa who walks these boards) and many more. And then there's the TOURING PLANS. They can shave 4-5 hours off your time in lines - they have ones for each park and some tailored to kids.
Birnbaum's is worth having - it has cool colour pictures, which will get you excited but once you read UG you won't need another guidebook for info.