Rock Band is wayyy better. First of all Rock Band has more instruments, the guitar, the bass, the drums, and the vocals. Also, it has way better graphics, and the characters on stage aren't all "cartoon-y" like in GH3. And also, the rectangle note capsules are way easier to hit than the circle targets. Rock Band has way better and more well-known songs. You can play with up to 3 friends in your living room or from across the world.
