I don't work with Overture personally on the PPC front but my sense is that depending on the product or service you're trying to push, that Google is most likely your better bet in many cases.
It seems that Google has achieved a higher level of awareness and brand loyalty than Overture has... I read somewhere that Overture is more used by those who work in the online/tech field than by your average consumer.
Having said that, they do populate Yahoo so that may no longer be the case.
If you're a merchant - you're best to ensure you're in both.
As an affiliate - I can't really say since I don't do PPC on the affiliate side but hopefully someone else will help you out on that front.