i found answers when i saw a bus outside the house.
backwheel drive is good and better. come to think of this, if you have a car with two motorized wheels at the back, and have the car fully loaded. and let the car move on inclined plane say about 30 degrees. where is the concetration of the force/weight? .. yes, physics comes in, but even with a simple look you know that as the angle of inclination increases the force acting on the Y axis increases. so to compensate the weight that would be concentrated at the back of the car, the motor should be at the back of course. another thing, it is always better to do turns with back wheel differential drive.