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We prefer AdWords because once you know the system it's just much easier to work with.
Overture has many built in problems that have been around for years.
Their interface is very slow and they are not as "up front" as Google.
They have many sleasy partners and you have to watch your serch terms like a hawk as they can get away on you.
If you are a small advertiser then avoid the high volume terms. Seek out terms that are much more specific and you should do well. Avoid content advertising until you know what you are doing.
It was a small study, and these figures are not all that trustworthy, but interesting non the less.
Use both! Google brings us a lot more traffic and seems more expensive, but in our space we can afford a high conversion cost.