The PokeCenter, and PokeMart are both half pokeballs, and the PokeMall is a full one. In the PokeMall there are 3 stations to buy and sell at.
On another note, the PokeMall is intended to be a MasterBall, but I couldn't find a good set of colors to do it... You can see my attempt at a masterball behind the PokeMart, but I wasn't satisfied with it.
PokeCenter
PokeMart
PokeMall