Generally none. If it were not so then a single-pole switch could not be used on an appliance with a reversible mains plug. Since electrical current flows in a closed circuit an interruption or break anywhere in the loop will switch the load off.
simulate this circuit – Schematic created using CircuitLab
Figure 1. Live-side and neutral-side switching.
Having the switch on the live wire is safer. If you take your hair-drier, for example, consider what the situation would be if it fell apart in your hand exposing the heating element. A switch in the live wire would isolate the element from the live feed whereas a switch in the neutral would not.