Google AdWords or Bing?
When it comes to advertising you want to be seen on the top results page of the search engine that has the largest target audience. In the UK and Europe that predominately means Google. Google has more than 80% of the Internet Search market (According to Statista Oct 2016)
Google Search is pre-installed on Android devices. Bing continues to grow, particularly with the release of Windows 10. Had their Windows Nokia phones been more successful they could have been on par with Google. Nokia is now set to release an Android phone which will fuel yet even more Google search results!
Bing does offer better introductory discounts to start advertising, but why would you spend your cash advertising on a search engine which could only reach 10% of your market?