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None of the payment options will give you a different ROI unless you don't manage your budgeting very well. Theonly difference is in the degree of control you have over how much gets spent and what degree of notification you receive.
On the non-stop Overture keep charging your credit card, so if things went crazy for a day or two it may be that you find you have been charged a lot of money (and hopefully made a lot too), but it could still come as a shock.
With the fixed budget, you will never spend more than you dictate, but may lose out on traffic that way.
With the pre-payment, you are in control over what gets spent and when.