I've split tested both in the past, about a year ago, and noticed that Adwords trumps Bing.
Bing just doesn't have the market yet. Maybe in a few years they will be able to compete with google, but right now they're not close.
Bing is cheaper, that's true, but you don't get as many clicks and they don't seem to convert as well even if they do click.
I was getting at least a 200% ROI with Adwords and at the same time I was barely breaking even with Bing.
I still like the bing/microsoft advertising platform much better because they support is top notch. They don't make you feel like a peon because you're not investing thousands every day. It would sometimes take me a month to get a response from adwords, and that's no good.