i think google adwords* since google has a larger volume of users + a larger user advertising database.
While Google Adwords does offer substantially more visitors to your advertisements than Yahoo or MSN PPC, they all have distinct characteristics. Critical among these is that Yahoo and MSN target different audiences. This means some products could actually see significantly more success on their services vs Adwords. There are also differences in the administration and rules of their platforms. You may receive more specific advice if you can provide basic information about your product (niche, demographics, et cetera).