Coaxial:
2 props above each other. 4x2 props instead of 8x1 prop
2 props above each other. 4x2 props instead of 8x1 prop
- Yes, the efficiency is reduced (because power usage per pound of thrust goes up).
A coaxial octocopter (what you call a quad with 8 motors) is about 30% less efficient (in terms of hover time) than a regular radial octocopter of the same weight.