Neurology covers more material than physiology, and in that sense is harder. It's also a little more black-and-white than physiology though (memorizing parts and functions of the brain and their various pathways through the body, and their blood supplies). Physiology is a little harder to visualize and grasp for some as a result.
In the grand scheme of things, Neurology took more effort simply because of the volume of information. But I find I have to keep going back and reviewing physiological principles because I keep forgetting them.