http://www.eurogamer.net/articles/digitalfoundry-2014-assassins-creed-unity-performance-analysis
Visually it appears identical on both consoles, but in the majority of scenarios that challenge the game's 30fps cap, our tests reveal that it is the Xbox One that takes the lead.
Frame-rates in Assassin's Creed Unity are far more unstable compared to Black Flag. Under load, neither version manages to achieve a consistent 30fps, but surprisingly it's the Xbox One game that delivers a smoother experience.
Given the immense graphical upgrade on offer, this isn't so surprising, but what comes as a bit of a shock is that the Xbox One version frequently out-performs the PS4 game in more detail-rich areas, such as locations littered with NPCs in the packed Parisian streets.