I'm assuming you mean AdWords, which is the search engine marketing product? Adsense is for placing ads on your own site.
With that assumption in place, the biggest difference between Google and Bing is the amount of usage each search engine gets. Depending on market and topic, Google can provide significantly more traffic.
Some people feel that the traffic on Bing may convert slightly better, but it really varies by what you are promoting. The demographics on Bing skew a little older and a little less tech savvy.
The costs are also hard to compare. Because google has more usage, you can target long tail keywords. Those same keywords might cost less on Bing, but don't provide enough traffic to really be worthwhile.
Also, the ads on Ask.com search results are (mostly) Google ads.
All four are popular ad networks which help target a huge chunk of the online audience. Google's search marketing services are preferred, so are Yahoo and Bing. It all depends on the level of prominence you want, on which search engine and at what cost. To gain relevance across search engines its best recommended to hire an interactive web agency like Resultrix, who take care of all our company's search marketing needs, just like they did for Virgin mobile, Corbis, etc.
There are many other ad agencies like Adbrite, TribalFusion, Stuckads etc but Google Adword program is of very high quality when compared to others. If you found an invalid/junk click you can report to Google and Google will return your money if they find any fraud clicks where as in case of others you will loose your bucks for fraud clicks however ad agencies will detect them but they won't repay you back. That amount will be taken by them.
When compared to Ask, Bing Google gets high traffic.