It's a bit more complicated. A 500W (which you can't get so we'll say 600W MH instead), would have better canopy penetration, as in the light would get better 'access' to the lower parts of the plants.
But 2 250W MH would have better coverage, as in because they're two lights, spaced further apart, they'd cover a larger area with more even and intense light.
Hope this helps, I'll get back in touch later on. Cheers mate.