PS4 is more powerful than XB1 full stop.
With that said, we won't know how much more "real world" visual difference there will be between the two systems 1 or 2 years post-launch until we get there. You certainly can't simply point at specs or the 720p vs 900p/1080p difference in two launch games and come to a lasting conclusion because there are many variables in play right now. I think ultimately the visual difference, if you could quantify it, will sit in the 5-20% range in favor of the PS4 after 18 months on market.
At launch, if you take a cross-section of 100 console owners out there (some dudebros, some 10-year olds, some hardcore, some girls, etc.) and had them play through the launch line-ups of both systems and then surveyed them on which system has better "looking" games overall, I think that number would be pretty evenly split. One the PS4 side, you have Killzone and sharper Battlefield/COD and maybe others, but on the XB1 side, you have Ryse, KI, DR3, Forza, and some same-looking multis weighing in on it's "graphical scale" so to speak.
I think a combination of the following factors (and more) have played a part in the discrepancy between COD/Battlefield versions: 1) console horsepower 2) resource allocation 3) state of technical documentation and support 4) timing of software/hardware kits released to developers 5) time/resources allocated to each version by devs 6) crunch time of hitting launch 7) XB1 being more complicated to develop for, etc. All of those factors are variable and will continue to change over time and are different from game to game, developer to developer. Will the visual gap stay the same or get larger over time? No one knows for sure. It's just speculation at this point. I think, just like now, there will be some difference, but most won't notice or care all that much (see 360 vs PS3)