Sizing DC Motors for a Two-Wheeled Balancing Device

[uglyoldbob]

I recently started doing research to build one of these (popular?) two-wheeled balancing things. (A segway clone)
I don't know how to calculate the proper size motors (a pair of motors) for this. I know you have to consider the torque and rotational speed required to size the motor.

Desired rotational speed is easy, I just take a desired speed and a wheel size and figure out how fast it needs to rotate to reach that speed.

I'm not sure how to calculate torque requirements.

 

[uglyoldbob]

Here are some motors I am comparing. The difference here is price (and obviously additional hardware on the more expensive one)

There is the CIM motor, according to the specs:
http://www2.usfirst.org/2005comp/Specs/CIM.pdf
Torque, speed, current, power, efficiency
0, 5310, 2.7, 0, 0 (free)
64, 4320, 27, 205, 63 (normal)
45, 4614, 19.8, 154, 65 (max efficiency)
171.7, 2655, 67.9, 337, 41 (max power)
343.4, 0, 133.0, 0, 0 (stall)

Here is a midwestMotion products motor: (it is the expensive one)
http://www.midwestmotion.com/products/brushed/12VOLT/12%20VOLT,%20250-324%20RPM%20GEARMOTORS/12%20VOLT,%2040-149%20IN-LBS/ENCODER%20-%20BRAKE%20COMPATIBLE/MMP%20S28-150E4-12V%20GP81-014%20TECH%20SPECS.pdf
Torque, speed, current, power, efficiency
0, 4140, 1.7, 0, 0 (no load)
110, 3640, 28.2, 296, N/A (rated continuous)

The more expensive motor has more torque from what I read on the datasheets. It is 3.2" diameter vs 2.5" on the CIM motor. What other big differences am I missing here (I put up links to the spec sheets of each motor)?