Apparently the US segment of the ISS runs on 124v DC, while the Russian segment is on 28v DC. Why the difference? I understand it is often safer to have several different voltages rather than one, but is one better than the other? Is the Russian segment lower because it is older, or because it is more efficient?
This answer is a near-exact copy of answer, by a NASA employee, given on Quora to the question "What voltage is used on the International Space Station" ?
The primary power at the solar arrays is at 160 V dc. That is transported across the ISS and then dropped down to 124 V dc at which point it is called secondary power. Most of the US equipment uses that 124 V dc. 124 V dc decreases the current and lowers the cost, weight, and power loss. The voltage is stepped down to 28 V dc for use by most Russian equipment and some visiting vehicles.
[My addition] The first-launched parts of the ISS, Unity and Destiny, were of original American design, so the choice of voltage could be made by NASA and its subcontractors. Coupling with Russian equipment came later. This answer to Soyuz MS spacecraft battery voltage? tells you that 28V DC is a common voltage in Russian vehicle, specifically Soyuz.