It depends on your industry. Most industries are doing well on Google. But some, I bet doing a lot better on Bing.
In particular, the highly competitive industries on Google and faced with huge CPC rates get much better results on Bing.
Another factor is the strict guidelines of both companies. In some cases where competition is tough and Google / Bing banned advertisers for silly reasons, such as frequent changes to page content, you can not help but move from Google to Bing or from Bing to Google.
I have been with Google for a long time because our niche is clean and the competition is moderate. I've tried Bing for controversial vertical applications like technical support, antivirus support, etc. with good results too.
My suggestion would be to do both. Spread your budget in a ration of 70:30 and put 70 in Google and 30 in Bing.
I do not include Facebook here because you have clearly asked for a choice between B and G.