Powerful home made DC motor...

Project for my new bot.
a 20" or so hollow center motor...
Basically a ring motor like most other pancake motors only without magnets.
I would wind the rotor and just apply power to make it appear like permanent magnets using two slip rings.
The inner stator would be controlled by one of my brushless pwm motor controllers.

So my question is this...
What design has the best power to weight ratio.

Flat coils with a stator above and below like a pancake motor,
Regular tooth design with coils pointing axially out like normal brushless motors.

Or one of those crazy Axial Flux motors?

Remember I am building this on a 20" diameter with no center bearing shaft.
I will use captive roller bearings like in a lazy susan.
Here are some links to pics of the three different types I am considering.


Axial Flux
http://www.infolytica.com/en/coolstuff/ex0072/ModelView.PNG
Regular brushless
http://www.infolytica.com/en/coolstuff/ex0088/BDC - Cogging torque - Full view.PNG
or flat coil pancake
http://sub.allaboutcircuits.com/images/02460.png

Thanks JC