Removing sugar from baking recipes
Sugar contributes to the texture in cookies. In particular brown sugar is softer than white because of the hygroscopic molasses. If you like your cookies soft I'd reduce/remove the white sugar first (plus you don't lose the brown sugar flavour). If that's too soft you can adjust the ratio in the next batch (or, to some extent, bake a tiny bit longer, but the texture really only becomes apparent on cooking) 

Baking time might change a little, so keep an eye on them when they should be nearly done.