It's not about resolution in 1080p in isolation,
a 1080p game just shows the platform had the power available for that resolution in combo with the rest of the game design decisions.
Each dev can make the best choice as to how to use what is available on each platform.
For multiplatform games, developers will simply have "left over" power on PS4 after achieving the same
gameplay features that XBone can handle, and they will choose to put that towards more resolution, FPS, effects as they best see fit.
It's about which is better: "more resolution/FPS/rendering details" or "less of the above"
XBone has to give up some of the above compared to PS4, because that's just the reality of their hardware,
that is why higher spec GPU's cost more, and why MS didn't go for a 33% smaller GPU than what they have.
If/when MS starts selling Kinect-less XBone's they can drop their price ~$100 on those bundles.
But if they want to free up the CPU/GPU resources currently dedicated to Kinect to be available for games,
that means that current and future XBone+Kinect2 owners will be unable to use those always-on
Kinect features while that hardware is "taken over" by a game utilizing it, breaking the expectation for Kinect.
This is on top of XBone's OS, which itself reserves (holds back from the game) more resources than PS4.
And yet XBone will still come up deficient in hardware power... Unlike last gen,
where each side had pros/cons, there is nothing that XBone's CPU/GPU can do that PS4's can't do (+more).
Heck, if anybody is truly happy with just the performance level of XBone, then PS4 games are free
to allocate their extra power to running calculations to develop a cure to cancer, AIDS, etc.