I agree with a comment here, you can not decide because a lot of factors matter. But, according to my results, Google is much better than Bing, and this for several reasons.
1st – Google has the most "real" shopping traffic.
2nd – You can easily dominate Google with long tail keywords, even in paid advertisements.
3rd – If you're in a micro-niche, Google will certainly have more targeted customers than Bing.