Well I think selling more is probably a result of producing more, and if Amazon is any basis of it, that seems to be what's going on right now. Sony may have had a better launch (who knows?) but the X1 is selling a more on Amazon right now. Seems like that's because they just keep restocking, selling out, restocking, selling out, while the PS4 doesn't seem to be restocking.
They allocated most of their early run to the NA launch and then the recent production pretty much all went to Europe, so it stands to reason that they haven't been able to resupply Amazon the same way MS has since their launch.