I love this game, I've put 60 hours into it already, no joke. However, I made a post a couple days ago in the thread asking people about this. BUT, I played the trial version of X1, and felt the game was really smooth fps wise. I ended up switching to PS4, because the X1 you can def tell it has jagged edges, and muddy/blurry edges (especially on a 55 inch TV). So the PS4's 1080p eliminates that and makes it look a lot sharper.
However, I also started to notice on the PS4 little jitters. They weren't like massive fps dips, but just little quick 1 second moments (panning a camera), or running through a city where it would have a quick jitter. I never remembered getting that on the X1 trial version, which I posted about in the thread.
Guess my hunch was right, and the X1 version is indeed more stable fps wise. On the plus side, the PS4 version is very smooth overall. The fps dips aren't substantial at all, and for the most part it looks and feels stable. But you CAN tell the little jitters here and there. It's not just in heavy intensive areas either. Like if you are running through a really ornate town, you will definitely notice these tiny dips/jitters or whatever you want to call them.
It's just hard to explain, because they are so small/quick that they don't actually slow your game down. I'm used to fps dips being more substantial so it's really noticeable, and feels awful. This feels more like a slight annoyance. I guess in the end, I don't regret going PS4. Just because I do have a 55 inch TV in my room. And I don't think the performance difference is night and day to the point that I would fret over it. But damn, good on X1.