Bing for iPad transforms a bland search engine into a visual application that lets you interact with information. The app is fluid, the integration of all the specialized search engines is almost seamless and there are a lot of small features that make your life easier. For example, you can quickly highlight matches, go to the next image result using gestures and go back to the list of search results using a back swipe.
Unfortunately, Bing's results aren't always great, but they've improved a lot lately. Voice search didn't work well for me and there's no visual search.
Google doesn't have a search app developed specifically for iPad, but there's a universal app for iPhone and iPad that adds features like voice search and visual search to the standard Web interface. The latest version of the app added two gestures that enhance the interface, but many users complain that they slow down the app. While Google's results are still better, Google has a long way to go to catch up when it comes to the user interface.