The difference in the 'rolling friction' of your two examples is not much and is way less they the basic requirements from Weight, Acceleration and Incline.
The other factor is that you need to design the motor/drive system with a good bit of extra power and torque. Running a motor at its full rated torque all the time is not good since it tends not to last long. (The exception is in some 'racing' or 'speed records' where the motor only needs to last for a short time.)
Also, study motor specs to understand what they mean. For one, they usually state the 'stall torque' and not the torque of maximum power output which is roughly half the stall torque.
Here is some good info on motor specs:http://www.micromo.com/motor-calculations.aspx