One of my fav. builds was Chinaphil's "Quest for Super Stealth". viewtopic.php?f=3&t=24866&hilit=chinaphil
Built specifically to negociate steep, winding, mountain paths in his part of China, and do it in a silent fashion, he mounted dual MXUS mini-motors.
See it in action,http://www.youtube.com/watch?v=WDm7lIHih8U
Not one to be given to exaggeration, he wrote,
QUOTE= " I always carry batteries (4 off 6s 5,000mAh 25C) in a backpack, easy to hear the LV buzzers.
Just climbed some 15% inclines and the speed is higher than the 500W Cyclone bike with all of its gearing - guess 15km/h versus less than 10 km/h - the inclines are narrow so there is no time to look at speed readouts.
At this stage I am content with the visual stealth, audible stealth, hill climbing ability and top speed."- END OF QUOTE
For technical, tight "trials" type riding, I'm thinking dual mini's would be hard to beat. Obviously, with this type of riding, the idea would be to keep both wheels on the ground. If a guy is "smoking" the trails, railing berms, jumping etc., yeah, a DD rear would be the way to go.
Splitting relatively low power between wheels creates a traction advantage, not to mention the small amount of added unsprung weight, less stress to the chain stay, better overall bike balance, etc.
2WD drive definately has it's place, I.M.O.
For low-powered assist riding, my FWD has been great, no control problems at all.