I, too, use a 28mm almost exclusively, don't even own a 45mm, just the 28mm and a 60mm. It cuts through 4 layers easily and cleanly, and will do 6 layers of average weight quilting cotton (meaning not Kona weight) with a fresh blade. I've cut circles smaller than DWR arcs many times with mine, templated and freehand.
The reasons I'd say to go with the 28mm instead of the 18mm are because it's easier to find replacement blades, it's far more versitile, and it's more comfortable in the hand.