It kinda makes sense that they differentiate the two; the career mode of pretty much every guitar game to this point has been very similar to the quickplay menu, except you unlock new songs by playing existing ones. Once you finish that, it really doesn't matter which mode you pick unless character selection is important to you. Ideally, they'd let you unlock songs through quickplay OR world tour in RB2, and let you complete the list whichever way you want, not unlike how Super Smash Bros Brawl's unlocking was done.