If you are going to market on ONE of the search engines, why not use all of them? In my experience, the clicks are cheaper and more valuable on smaller search engines, like Ask.com, than they can be on Google. Granted the traffic is less which means less impressions, but the clicks are still there, trust me. Do you have any experience marketing on any search engine other than Google?
I have tested Google , Yahoo and MSN and have found that by running the exact same ad I will get a better conversion ratio with Yahoo and MSN although the traffic is much less
Google will ultimately give you the most traffic and the most exposure. However, that doesn't mean you have to ignore the other search engines. But I would definitely give the most weight to google.