to tell you the truth, i think it's important to be able to do a bit of both. Just because you pick one style, don't drop the other one O; continue on with it too!
since you said you're already good at the cute style, if i were you, i'd work more on the serious style-- or even just change the medium you're working with for a change.
if you want to keep to the cute style, try using different mediums-- pens, pastels, paint --acrylic/watercolour :) it's fun to mix things up once in a while