Essentially zero - technically the PS3 was more powerful from a computational standpoint which was why it saw so much use being strapped together by researchers but from a videogame perspective it was essentially a draw with PS3 having marginally more power if coded for properly.
The main elements were that both had 512 MB memory but different designs with the memory being split in PS3, the 360 had a smaller OS footprint freeing up more resource for developers and a stronger GPU, the 360 was easier to develop for and had better SDK (particularly early on) and finally the PS3 had Cell and allowed better "coding to the metal".
Taking the above together the end result was that the PS3 was harder to code for and hence for a fair time multi-plats looked significantly better on 360 (and I do mean significantly) then things started to balance out as developers were able to leverage Cell to where we ended which was most multi-plat games being either at parity or only slightly better on 360 and PS3 full exclusives actually outstripping best performing 360 titles a notch (not much but enough to show that fully leveraged the Cell did in fact give the PS3 a slight edge even with the 360s own advantages).
It's important to understand that PS3 / 360 were essentially identical but PS3 had design choices that greatly slowed developers in achieving same results as they did on 360 hence why the PS3 was able to "catch up" overall and even out-perform the 360 a little with 1st party exclusives.
This gen it's totally different though.
- The PS4 has an easy to develop for architecture which I'd argue is easier to code for than XB1 but would accept a worst case view that they're equally easy to develop for - this is a big change from last gen
- The PS4 has a much clearer and stronger power advantage vs XB1 and there is no Cell like element of XB1 that over time will allow for a claw back
- The PS4 allows better coding to the hardware than XB1
- The PS4 SDK actually seems ahead of XB1 at this point
Taking all the above into consideration plus what we saw last gen its pretty clear that anyone with computing / coding experience (as I do for example) would expect PS4 to start slightly ahead and likely move further ahead in terms of graphical differences.
So unlike last gen there is no sign of anything (other than normal SDK improvements and OS and driver improvements which PS4 will see too) to suggest any way for XB1 to catch up as the console is factually weaker not essentially the same just harder to code for - hence why I don't believe the XB1 will "catch up" the same way the PS3 did simply that it will improve in line with PS4 with a gap always remaining in capability.