It seems to be a fairly clear-cut geographical divide - Americans always pluralise, Britons never do. Without knowing but based on other lexical splits, I'd exp

It seems to be a fairly clear-cut geographical divide - Americans always pluralise, Britons never do. Without knowing but based on other lexical splits, I'd expect the majority of the rest of the Anglophone world to follow British usage, with Canada going either one way or the other. Usually Americans can cite 17c usage as a precedent - not in this case !