DuckDuckGo is forging a new path in relevance. Results returned include a wide array of different possible answers to what one is searching for, rather than a lot of content on the most likely item. A very disruptive approach to search.
The site is quite usable. It emulates much of Google in the simplistic home page and the general structure of its results page. However, display favicons for the sites sourced as results, showing more than 10 results by default, and by providing additional info along with the linked results makes DuckDuckGo a leader in usability.
The best. No tracking. Period. Offers the most private search experience currently available to the average Internet user. Also has very robust settings for changing relatively low-level details about privacy options.
Basically no ads. Some times may link to a product, which is likely earning affiliate commission.
Solid mobile experience. Nothing special but they get the basics right across mobile, browsers, and of course, the home page.
Bing is known as the number two search engine behind Google, and so does the data (i.e. search volume on either platform). Bing has also been accused of copying Google search results. While often highly relevant, Bing Search plays second fiddle to getting the information you need faster through Google.
Microsoft and Bing have come a long way in improving the user experience. Bing highlights the first search result with a box that has several options (i.e. a search for say, Google, returns the direct link, stock information, and more). Also their homepage creates a pretty pleasant experience with easily tabbed search options, realtime search trends, and a nice touch in beautiful background images.
While Bing has not gotten the public privacy backlash that Google has faced, its privacy practices are not necessarily vastly different. Location is also instantly identified when performing searches, and queries are indeed tied to IP addresses, or MSN accounts if you have one.
While Bing also serves ads adjacent to search results, ads on Bing appear less intrusively than even Google and its relatively non intrusive ads. Specifically, ads only appear to the right of the results, and not on top of the organic results.
This is an area in which Bing falls short. While available from the web and many browsers, Bing has not mastered the mobile experience and renders rather poor interfaces and results on mobile devices that are not Windows-based.