The advantage of the walling hammer is that it's only one tool. (The fewer tools you are using the fewer you have to lose) The walling hammer is efficient. It's simple. It's fast.
The hammer and chisel on the other hand can take off lumps that are almost impossible to get with the even most deftly swung hammer. I've got good at both methods and I still believe this to be true.
The hammer & chisel method will always be more accurate too. The walling hammer is a wee bit more likely to wreck a stone. Granted a good waller could still use the misshapen stone in the wall but sometimes you just need to make sure you re not gong to ruin that last perfect stone you've found for that special spot in the wall.