I'd put my money on google. I have some top 3 bids at goto on pretty competitive terms and I don't get anywhere close to the traffic I get from google.
I think it may be hard to say who actually gets more searches but counting referrals is pretty easy.
Take an engine that incorporates Goto top 3 into their first SERP - you still only have a 30% chance that you'll get a goto related referal - assuming the SERP is 10 results.
Even if goto had more searches the way they are incorporated into some of their partners lowers the odds that you'll get a referral based on your goto bid but at the same time it still counts as a goto related search.
a little late here, but figured i'd give my 2cents. we have plenty of goto listings for a host of terms, but i'd have to agree with others here - google is where the traffic's at.
that said, the beauty of goto is that you can get started there fast, and start bringing in traffic immediately. google could take anywhere between 6 weeks and 4 months to start hitting your stride...and of course, there's no guarantee of the results.
in the meantime, you could identify your major traffic terms and take advantage of PPC opportunities the get the traffic started. thats how we generally approach it.