I'd go for CA or epoxy glue by all means. PVA doesn't bond with polystyrene, so while it might hold together for some time, the miniatures will eventually fall off.
if one of the two sides is plywood and the other side is thick enough, pva would hold for a century before falling off... i wouldn't worry that much, personally! And scratching the surface before applying pva will help further...
But if you have epoxy glue, why not? the problem is that's more expensive, and for a large area could be a cost...