Either, depending on the game.
This.
Games I'm really looking forward to, I buy right off. Because I'm, well, really looking forward to them.
Games that I'm somewhat interested in? Depends on whether I've got current games to play or not. (like, if I'm currently in the middle of something, no need/reason to get it right off.)
So far this year, I've gotten several games right at release - Final Fantasy 13-2, Kingdoms of Amalur, SSX, and Mass Effect 3. (Which, honestly, is alot of games for me. I don't generally buy that many games in a quarter. Last fall/winter, for example, the only "new"/current game I got was Skyrim.)
Oh, and I only had trade-in credit towards one of those.... I didn't see playing FF13-2 again anytime soon, and Gamestop had a trade-in special for it, toward SSX.
Games that I get on sale? Frequently are impulse buys that I get
because they're on sale - like several really cheap things I got during the Steam winter sale.
edit: upcoming games that I'll probably grab right away - Diablo 3, Borderlands 2, Bioshock Infinite