Definitely Google. ;) Joking aside, I'd try them both. They have different strengths depending on what you want to do. One nice thing about AdWords is the ability to limit clicks to certain countries. I'm sure Overture has nice features, but the people over in the Overture forum would be able to provide better plugs for them. :)
One good thing about Google is that their ads are better separated from the "editorial", "free-indexed" main index. With OV you are not looking at users clicking from their own site but from many sites that use their results and display them with varying degrees of transparency that they are indeed "paid for".
That could mean that people that btob buyers are seriously looking for services may often go to google for the express purpose, rather than "stumbling over them" in MSN, Y! AV etc etc. Business professionals seem to be more targeted than general consumers and want to get results fast (thats their job), so they probably appeciate having the commercial and info results separated and will look straight to the right in Google. They dont want to waste their time shuffling through info sites, (they probably already know their industry anyway), to find the buy button.
That said its a great point that otehrs have made that it depends on the sector. Do what we do and use both! For Europe i hear (only from here) that another PPC is worth a try too (cant remember the name).
We are an affordable product alternative company. We find because of our niche in our market that Google generally likes our site and pages, and is the better search engine for us. In our niche, people that search via MSN for instance are not keying on "affordable" as much in their purchasing decisions.
Our observations indicate to us that Google seems to have liked the word "free" alot when indexing new URLs. It seems to also like web sites that are intensly informational (the phrase "help" comes to mind).
We don't know if Google weights certain keyphrases higher than others when scoring URL's. It could be our imagination or other reasons giving us the impression.