Aluminum bikes are almost automatically 2lbs lighter than the same size steel bike w/ similar components. You will have to spend quite a bit to drop 2lbs on non-frame/fork components on a steel bike.

Getting a bike down to 18lbs is easy. Getting it to 16 or sub 16 is exponentially more expensive.

Fit is more important than weight, but if the Langster fits you better, then you get fit and a lighter bike.