Depends what's coming out of your mains. The lamp voltage is the voltage where your lamp will give you its rated color temperature. All dimmers drop a few volts, and linevoltage varies quite a bit, but for example, if your mains voltage is 120 to 125, then the 120 volt lamp would be the one. If your mains voltage is below 120, then the 115 volt lamp would be better. If you are not too concerned with color temperature or peak output, then the 120 volt lamp is going to last longer in either case as it will be under less stress.
The control electronics usually allow you to set output voltage per ea. dimmer.
Thus you can set certain circuits that remain the same (houselights) at a defined output voltage, say 120, or 115 if you want to save on lamp life.
My Sensor CEM+ system easily allows this via the web browser connection and I have certain fixtures that I know usually have S4's set to 115v, others with long life PARS set to 120v.