I like Google and never tried out Yahoo!. I think Google is all the way better but it has a nice feature of transfering your money to invest it on Yahoo! search. Google will surely come up with something as it's currently working on Gbuy which might prove to be Paypal's rival. I think it's going to do well and if it supports GBuy (Money Payment) google might get a bunch of new publishers
However, Google for me all the way.