Forgive my ignorance but is there a difference?

Only difference I can imagine is the shape of the tyre bead / rim section. But both BST and UST say you can use any type of tubeless tyre, so that can’t be the case.

If there is a difference, which is better if either?

Theoretically no difference.

Practically, Mavic UST rims are less faff to use (no need for rim strips or any of that cobblers) and seem more robust. Stans BST rims are a lot lighter, but take much more effort to inflate and seem more dink prone.