Originally Posted by wphamilton

Same weight, the difference in rolling resistance will be trivial. If any, because the rolling resistance is proportional to weight (and speed) so while more wheels can give more resistance each of those wheels has less resistance in proportion to the lesser weight on each. The mechanical losses in the extra sets of bearings is almost nothing.

At the speeds mentioned, 6-12 mph, air resistance is not significant enough to be concerned about any delta. So the power requirements will be very close either way.

Handling and balance may be affected, which may make the rider less efficient on one setup or the other. My "physics" guess is that this is where the only real difference in wattage is found. Unless you want to consider higher speeds.