slowrotor.
delta3's points are good ones.
A couple of additional comments.
Electric motors can deliver 2 to 3 times their operational power for a short period of time. This might be used for safety.
In general, bigger motors are more efficient. However, there are small outrunner electric motors that have higher power/weight ratios then bigger ones, but their reliability is less.
You might find this Patent Application entertaining;
20060266881 ~ Vertical takeoff and landing aircraft using a redundant array of independent rotors.
This might also be of interest;
Electrotor-Simplex _____________
Now that the fingers are warmed up. Your concern with safety has provoked a thought.
I believe that a rotorcraft that has extremely strong bilateral rotors, should be able to fully stop the rotation of the rotors and then, 'hands-off', allow the stalled craft to pitch nose down, regain forward velocity, regain rotor rotation (w/o engine power), and then be recovered from the dive; just like a fixed wing craft.
Perhaps building
this model, without the rubber-band, and then dropping it from an elevation might prove the belief.
Dave