I am planning to use this motor: http://www.adrirobot.it/robot_kit/feetech_FT-DC-002/TGP01D-A130_TT-Motor.pdf Model: TGP01DA130 12215-48. But I have some questions: How much a robot will be slower if it has 1 motor and one 180 degree servo instead of just 2 motors? And which one is the best option?
If you want to use the second motor/servo for steering, I suggest you used a servo. To me, It's easier to operate for steering and more accurate.