Is there a specific approach for determining best paid search advertising?

I have to determine which is the better format to advertise on considering their market share and price of key words. Obviously the one with better ROI is the one I would prefer. I believe testing would be the best route, but does anyone have a breakdown or advice of how to go about it. How much should I invest in the testing? At what point can I make a clear decision? Anything else I should be considering?

Thank you!