Light removes the advantage of elves have against Lizardmen (high WS, BS, and I) by evening the odds. Life capitalizing on the weakness of High Elves (low S and T) by making your troops harder to wound and killing droves with Dwellers. I can't figure out which I like better in general, but extenuating circumstances push me towards Light. My most frequent elven opponent is a big fan of the Lore of Shadows in general and against LM in general (because of our low Intiative scores). Speed of Light with a 12" radius defends against both Pit of Shades and the Penumbral Pendulum quite well.
Personally I like light as it takes away their re rolls, and because +4 earth blood isn't any better than +2 vs those Girly elves