I have a 28mm and a 60mm, and use the 28mm for almost everything I cut. Someday I'll treat myself to a 45mm, but only so I can use the decorative edge blades for art quilts. I really, really want a circle cutter, but Santa has forgotten where I live so it'll be awhile before I see one of those.
If you think you'd like a smaller cutter than what you have now, you might want to consider the 28mm instead of the 18mm. I love the control I have with it and it easily cuts curves, too.