There is a slight advantage to 220v over 115 but not that much. I guessing that it amounts to less than 1% energy savings. The manufacture should be able to give you the energy efficiency difference. However, for most pumps that run on both, the 115v max current is twice that of the 220v so even the manufacture does not bother to take into account the efficiency differences. I am not even sure that the cost of installing 220v is covered by the efficiency difference.

The other difference is wire size. In some cases, 115v will require heavier guage wire (more current). I think that this is the most important factor and why higher HP motors > 2 HP do not come in 115v versions. Even at 2 HP, the 115v 2HP Northstar requires 10 AWG wire which is very difficult to work with.