I'm left-handed. Yet when it comes to computers, like most other lefties I know, I use a mouse with my right hand. It's just how I was taught. So I've never seen the need for a mouse that tries to accommodate both normal humans and sinister mutants.
But this category of the market exists because people want that flexibility anyway, so if you really must sit on the fence, the Avior 8200 is a good way to do it. Made by Mionix, the Avior is a fantastic gaming mouse that's also subtle in design. There are no enormous logos or flashy color schemes, build quality is superb with a nice feel to every button, and the soft matte finish to the exterior is extremely comfortable.