It depends on your industry. Most industries do well in google. But to some, I bet Bing a lot better.
Especially the industries that have fierce competition in Google and face huge CPC rates get much better results in Bing.
Another factor is the strict guidelines of both companies. In some cases where competition is harsh and Google / Bing advertisers are for silly reasons, such as changing the content of the page frequently, they have no choice but to switch from Google to Bing or from Bing to Google.
I've been with Google for a long time because our niche is clean and has moderate competition. I have tried Bing for verticals in dispute such as technical support, antivirus support, etc. with good results too.
My suggestion would be to do both. Divide your budget into 70:30 servings and put 70 in google and 30 in Bing.
I'm not including Facebook here because you clearly asked for a choice between B and G.