Google hands down. But you can't ignore Bing advertising if you can afford it too.
I (personally) have ran some sizable campaigns in the past and found that whilst you have no issues spending money on Google. Spending money =people who see your ad. In certain countries Bing takes up a really low share of the search engine pie. This meant that Bing ads sometimes don't even trigger due to not finding an audience. I found this particular true with paid advertising in Germany.
It was no issue getting impressions on Google in Germany, but Bing barely triggered as it barely found the same audience.