What is the difference between the 2 please?
They seem to be the same... except maybe for the extra torque offered by a servo.
How would an infinite rotation servo act when added to a servo controller (say Pololu)?
They are very different animals. A stepper motor is a motor with a finite number of positions each revolution, while an R/C servo, whether infinite or limited rotation, is a combination of motor, gearbox and motor controller.
Steppers are drven by energizing the vindings sequentially (controller needed). The servo is controlled by a PDM signal (1..2ms pulse repeated each 20ms).