"People with astigmatism (approximately 50% of the population) find it harder to read white text on black than black text on white. Part of this has to do with light levels: with a bright display (white background) the iris closes a bit more, decreasing the effect of the "deformed" lens; with a dark display (black background) the iris opens to receive more light and the deformation of the lens creates a much fuzzier focus at the eye." - Jason Harrison – Post Doctoral Fellow, Imager Lab Manager – Sensory Perception and Interaction Research Group, University of British Columbia

This quote describes the situation well I think, why the opinions differ and why neither dominates UIs really.