I`ve been using both PPC systems in in site WWW.Bookingsonline.net.
At the yahoo system I had a lot of fraud clicks, about one or two page views per click and also tens of fraudulent bookings on my site, that with no exceptions had been canceled (The site permits free reservation and cancellation).
In this case I am convinced that the fraudulent clicks and bookings were done not only by the public, but olso by someone related to the yahoo. (I hope I am wrong).
In relation to the Google PPC system I am using the daily limit of 2 USD and in general it is working weel. I have about 10 page views per clik and 1 booking per 20 visiters. Some days it seems that I have fraudulent clicks, but it is not usual. The Google team seams to be working in a very professional way in relation to this issue and are begining to convince me to put some more money on it.