Well said, webdiversity. Each of those PPCs have their own strengths, and weaknesses.
The demographics they serve are also vastly different. OV has Yahoo which are very different users overall than G users.
You are not just using the PPC, you are using their distribution network. Knowing how to attrack each demographic plays a large part in the success of either one.
If OV suddenly switched to showing ads like G - I would not use my G ads - those ads are created for the G demographic and not Ys users.
The key is to take advantage of the strenghts, and try to minimize the weaknesses of each ad stream you use.