120 new "challenges" have been added to the games in Bit.Trip Complete, along with new difficulty modes and leaderboards. In addition, there's an "audio gallery" of new songs, demo versions, and fan remixes of the series' signature sounds. Other bonuses include image and video galleries and a bundled soundtrack CD.
It'll be up to you how you want to experience these games -- handheld and in 3D, or on your TV with bonuses. You could always buy both in order to have the most options as to how you want to be mesmerized and frustrated.