As others have said, it's up to you. Personally, I prefer supporting the developers before Gamestop, even if it means spending a couple more dollars. Gamestop has, of course, a financial incentive to sell used. Here's a case:
Take new HD game X. MSRP (and Gamestop's price) is $60. Wholesale on this game is $48. (Statistical average of wholesale price.) Gamestop makes $12 on this game. However, someone has traded in this game, and gotten $30 credit for it. You save $5, and buy it for $55. Gamestop now gets $25 profit, more than double the new case. In worst case for Gamestop, said game was traded in at a 30% extra promo (most likely toward a new game), so $39 was paid out. You have an edge card, and buy during a B2G1 event. Assuming you buy 3 games at $55 each (getting one free), less 10% on the remainder, means you've paid $99. Gamestop paid $117. Whoops, Gamestop lost $18!? Of course, this scenario is not the norm. (However, the 3 new games each sold from the tradeins at $12 profit each keeps GS up $18 after all is said and done.) Even if the games were just traded in straight, they still make profit on you, albeit not as much. But the potential for profit is there, and greater, so they're going to push you that way.
Ultimately, it's up to you. If you value your personal dollar most, get the used copy. If you want to tell the developer good job, and give them the support, get the new copy. Only you can determine which is right for you.
-dunno001
-On a quest for the truly perfect game; I don't think it exists...