What I'd love to know is how the X1 sales stack up against MS's internal projections. While it's useful to compare yourself to your competitors, the main benchmark for them will be how their system is performing against how they expected it to perform. That's likely to be a more significant factor in their decision-making than the performance of the PS4 (though of course that will be a factor). MS also have to contend with the perception aspect - given they have made some significant changes in recent months in response to the public's feedback, they will be torn between staying the course according to their vision for the console, and bending to public opinion - ideally hitting the sweet spot between the two that maximises sales.
There are a multitude of possibilities for how they proceed - they could do nothing, and remain focussed and confident in their product and remind everyone of their oft-repeated "in it for the long haul" mantra, and trust that their customers will be open to paying out $100 more for their machine. They could cut their price and take a loss on each machine sold, which could be an option given Xboxes traditionally have a high attach rate (ie aim to make their money back via software sales - but this will also require investment in first party studios, which IMO MS could do with more of). Given the recent outcry about selling off the Xbox division, cutting the price probably wouldn't go down well with shareholders and investors, given the 360 fiasco. Maybe they will take a middle of the road approach and stay with the same price but bundle some freebies in (games, gold subs, extra controller, etc). Obviously all speculation, we can't know what decisions they're taking internally, but interesting to think about anyway.
One thing's for sure, Sony must be thanking their lucky stars the PS4 is such a success, given their otherwise disastrous performance of late. In terms of how either company performs as a whole, it looks like the PS brand counts for a large number of eggs in Sony's basket. Although the Xbox brand is significant, MS would remain highly profitable even without it, so from that perspective at least, MS are in a more comfortable position. Even that's not necessarily a positive though - MS's success with the 360 and their apparent view that the One was preordained to be successful seems to have resulted in a somewhat bloated piece of hardware filled with potential but which hasn't yet found its niche, whereas Sony clearly learned from their failures with the PS3 and have come back with a streamlined offering that knows exactly what it is, and the paying public have responded accordingly.