if you like to shoot in ambient light, outdoors, definitely lens.
but if u prefer indoor shots, even if you think you like ambient light, a proper flash is far more useful. since you are asking this question here, i don't think you have tried bounced flash before (i don't mean to be patronising, so sorry if i come across as so).
use a flash with a head that can rotate and tilt, and u are in business. your kit lens is very good and sharp liao :thumbsup:
my recommendation is SB 600. don't go for lens first. maybe the next thing after u get the flash is 35mm AFS DX from nikon :thumbsup: