There is no question (it is not debatable) that PS4 GPU has 32 ROPs while XB1 GPU has 16 ROPs. PS4 GPU is clocked at 800 MHz, XB1 GPU is clocked at 853 MHz.
PS4 therefore has nearly twice the pixel fill rate. 25.6 billion pixels/sec vs 13.6 billion pixels/sec for XB1.
pixel fill rate is absolutely important for both resolution and framerate.
Aside from ROPs, PS4 has more Texture Units (72) vs XB1 (48). on top of having more shader cores (1152 vs 768).
So while PS4 only has about 40% higher GPU shader/ compute performance, it has nearly double the pixel fillrate
These things were set in stone forever for the lifecycles of these consoles once the hardware was locked down. None of the performance boosts for XB1 (i.e. freeing up resources once used by Kinect, or the upcoming DX12 API) will make up the deficit because the gap between their GPUs is too much.
Does that mean XB1 cannot do 1080p ? No, of course not, but it means that if any given XB1 is 1080p, cutbacks will have to be made in other areas, namely framerate, or graphics quality / effects, etc. compared to the same things on a PS4 ver.
If the XB1 version of GTA V ends up being 1080p with the same graphics as the PS4 version, then the frame rate will suffer.