I do not own one, but me personally I would go with the medium. You can cut down on the coal for smaller cooks with med BUT can only add X amount of coal in a small vortex for a large cook.
To me it is kind of closets in a house you never have enough of them OR bullets in a firefight.
I do own a kettle and like it alot.
My daughter bought me PKO and a Burnshop charcoal basket and the girl thought ahead when she ordered the coal basket and ordered 2 dividers. What this does for me is I cant sort of immulate a vortex. But I have the option to choose the size of coralled coal from extremely small to medium. I use the medium size the most, however remember I am talking PKO not kettle.