I almost always go with used. It's just not fiscally feasible for me to buy every game new, apart from franchises I really, really like (Zelda and Pokemon) and from the oddball game that catches my eye (like Portal 2 back in the day).
Beyond the fact that I just don't have that kind of money, I just don't care for most new games these days. It's like the industry has been infected with ultra-gritty "realism" and it's spreading to everything. Half of the games I see in the stores are first-person shooters, and apart from Bioshock Infinite they just don't have any appeal to me. (Though if I'm being honest, I do enjoy a few rounds of Zombies in Call of Duty, but that has more to do with the fact that it's with my sister and her friends, and it's really fun playing with them because they get scared easily.) I'm much more into the adventure and platformer genres and story-heavy games, like The Walking Dead, and so those tend to be the only games I get new or at least from within the past few years. Older games, my tastes are all over the board so it doesn't quite matter as much.
Also, I intensely hate the crap that developers pull on their games nowadays. I mean what the hell, why do I have to be online to play a single-player game just so they can run their stupid DRM crap? Also, they have gone overboard with the DLC in my opinion. Buying a new level or character to a game (which wasn't already on the disk) is perfectly okay for me, but not most of the other stuff. *rocking in my crickety old chair* I remember back in the old days when you unlocked a new level or cheat by, you know, PLAYING THE FREAKING GAME! *slams cane against the wooden porch*
I will admit, though, that my buying habits make me a bad customer for everybody but the developers for Zelda and Pokemon. Most times this doesn't bother me, but a few times it has gotten to me. I didn't buy Okami new back when it was re-released for the Wii, nor did I buy Chrono Trigger when it was re-released for the DS, and so those people will never get their hard-earned cash from me. And that really sucks, I WANT more games like Okami and Chrono Trigger coming out instead of the drivel that I see nowadays. Other games I regret not buying new to support the developers include: Metroid Prime 3, Donkey Kong Country Returns, and the Metal Gear Solid HD collection for the 360.