I prefer symmetrical. It just works better in my head. I have a third-party controller for my PC where you can switch to either one and I just don't like asymmetrical for FPS. TPS doesn't really matter much.
Honestly, though, it's not a big deal. It's just which one you get used to. If this is a deal breaker for anyone then I hope they never play any older systems. I've nearly caused permanent damage to myself with the Atari joystick and the NES torture device, two sticks being in slightly different places means nothing.
Also, before anyone starts that "prime placement" nonsense, the 360 and PS3 controllers have different shapes, where your thumbs fall naturally is different. Not that a couple inches either way matters. I don't care if you staple some buttons to a dog, I'll still play on it.
