PCB Design and Power Traces / Plane
I would go with polygons for 2 reasons:

1) More copper = more area = less thermal problems due to current heating up the copper.

2) By filling the entire layer with copper, regardless PWR or GND, you are reducing the gaps that the prepreg can flow into when the fabricator presses the layers together. This helps keep things mechanically stable and ensures an even thickness of prepreg across your board.

Here is a useful calculator for calculating required trade width given copper thickness, current, and temp rise: http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/