The Wand directs the will of the wizard. It's masculine in nature and represents that.
Anything phallic is supposed to stand in for the symbol of cosmic power, as acted upon by the will of the wand-holder. Sword, athame, rod, sceptre... branch, wand etc.
A staff is like a giant phallic object that looks cool but I think it is more useful as a walking stick than a ritual implement.
Unless one has 'size' insecurities, if you know what I mean.