For all practical purposes in this case. The heat generated is more affected by the amps going thru it. So whether you are putting 120 volts or 240 volts, the heat will be about the same because the amps into the charger are the same.
In a purist sense, the higher voltage input would give you slightly less heat, because there is less 'work' that needs to be done with that electricity to make it match the input voltage to the battery, however that is partially offset by the fact that the lower input voltage, although more 'work' is needed to make the voltage right, fewer overall amps are flowing out the ass end, and since that amp throttling is on the INPUT side of the charger generally this tends to balance out.
I have charged my Ribelle, which I believe uses the same basic type of built in charger for AC charging, and whether using 115 or 240 or something inbetween... using a thermal imager, am seeing the heat is really not that much different between the two charging voltages after charging for a few hours.
Aaron