18 tooth is supposedly 11-32, 20T is 11-34 from what I've read.
90% of my off road riding is local bridleway bashing, and I hate spending as much time cleaning my bike as I do riding it. And I hate riding along listening to chainrings, cassettes and jockey wheels getting ground down by the mud. I have come to the realisation that however much I love my Soul, given the riding that I do off road, an Alfine'd Inbred would be a much more practical solution. And then I could bling up the Soul for those rare "proper" rides.
Mind you, most of my total riding is commmuting by road. Now you can get drop bar shifters for the Nexus hubs, I can see a solution to the winter salt and grit problem on that bike as well…..