The Fender Squier
is a real guitar. On a promotional event a guy had it hooked up to an amplifier while also being connected to the game, so you could actually hear his playing while the tune also rolled in Rock Band 3.
Check the (rather cool) demonstration/teaser video on Fenders website.
In other words, if you go for the Fender Squier, Rock Band 3 will function as a learning tool. That games career mode is more about challenges rather than just plauying at venues. Different difficulty levels (where the hardest setting equals the way the guitarist is actually playing the current song), tutorials and the ability to slow down sections at will makes it possible to get down to business learning how a guitar produces sound. If you take the time, you will be able to learn to play. The only thing the game doesn't teach
specifically is technique, but with the songs including (and the game supporting) hammerons/pulloffs, left hand muting, open chords, arpeggios, sustained notes and slides, the techniques will come eventually as long as one is eager and willing to learn.