The difference in terms of graphics potential between PS4 vs Xbox One is greater than that of PS3 vs Xbox 360.
Note that I said graphics potential. The PS3's Cell was very hard to work with. The architecture was better suited to a workload a server would handle, not a games console. Using the Cell to power the PS3 was like using a jet engine to power a car. Yes, the potential was there, and it had insane numbers, but it really wasn't practical. And some developers, such as Naughty Dog with The Last of Us, showed us just what it was capable of when used right.
There's a combination of reasons behind the percieved difference this time around being less. The first, is diminishing returns. We've gotten past the point where, double the power doesn't mean we'll notice image quality twice as good. The second is the media. I remember hearing all bashing the PS3 got because it ran games at a 10% lower resolution, or had a framerate a bit less stable. Fast forward to 2014, and now we're being told that the difference "isn't that big" or "You won't notice it"
"Never argue with stupid people. They will drag you down to their level and then beat you with experience."
-Samuel Clemens
