Just based on collection area: You would want the tarp to be as flat as possible and still direct the water. It's not so much surface area but projected area in the direction of rainfall. Cone or slope shouldn't make a difference, but for some reason I want to think the option with the center drain would be easier to setup. One hopes the rain is mostly vertical and not like I've been seeing the last couple of days - remnants of Gustav; 8.5 inches of rain total and 4 tornado warnings with 2 local siren activations today.

-Tommy