i think it's kind of trivially obvious that it's both. clearly some people who own one console would own the other if it was the only choice. but also clearly at least some percentage of owners chose to "jump in" to either system for some exclusive or other.
the thing is -- it doesn't really matter--developers and publishers have signaled that the japanese market means less and less to them as the western market means more and more, so it's not likely development or platform decisions, either way, will be designed to maximize japanese sales.