To my eye, 220v is better, but it doesn't make much difference for smaller motors (less than 1 HP). Even though there isn't any difference in inherent efficiency, there is often a difference in practice, especially with long wiring runs between the electrical panel and the pump. JohnT is correct that you can compensate for this by using larger wire, but this is not always done correctly in practice, especially with motors over 1 HP where the required wire size is very large and difficult to work with.
Even though there doesn't need to be any difference in efficiency, if you look at real world installs of larger motors, you will generally find that, on the average, the 110v installations lose more energy in the wiring than the 220v installations. If your motor is under 1 HP, I wouldn't worry about it. But larger motors there can be some practical advantages to using 220v.