Your hands typically remain in the same place on the steering wheel when you're racing, so having the paddle shifters mounted to the wheel, so that they are at the same location every time you click the button makes more sense. It would suck to have your hands wrapped around for a hairpin left turn, and you can't upshift or downshift, because your fingers aren't near the correct paddle.I thought paddle shifters were supposed to mount on the column and not the wheel… ya know, so up and down shift are always in the same damned place. I think I’ve used them twice just trying it out.
If you want it mounted to the column, go with a ring where you pull it towards you for an upshift and push it away for a downshift. I believe Tiger Racing did that for one of their road course cars a few years ago, as they had a handicapped driver.

