Using both Google Adwords and Overture for my clients, I am more in favor of Overture. Although Google does have tremendous reach, Overture is much easier to use.

The billing is handled a lot nicer. With Google, you never really know when you will be billed and you are unsure exactly what you will be charged on your credit card. Overture allows you 3 different billing options to fit your needs. My favorite is adding funds at your leasure. This works great when I need to wait for my clients to pay me.

In Overture your ad is always showing up unless your funds run out or you make your ad "offline", With Google, I have had numerous complaints about my clients ads not showing up when they type in the specific keywords that they requested. How do you explain to your client that for every 15 times that keyword is typed in, your ad will show up, unless you are willing to pay more money?

Overture easily allows you to customize your ads specifically for each keyword. Google makes it a bit more complicated of a process to customize each ad on a per keyword basis.

All-in-all I prefer Overture, although I still use Google only because of its reach.