Level shifting 3.3V to 5V for PWM output to servo
To answer your specific questions for this task:

MOSFET - Pros: None Cons: High gate capacitance could lead to low switching speeds.
Optocoupler - Pros: Electrical isolation Cons: Multiple components
4.7k Pull Up - Pros: Low cost and easy Cons: Only applicable if MCU has 5V tolerant inputs

You could also try option number 4: Use a discrete logic IC for voltage level translation e.g. NC7WZ17 - Pros: Low cost and easy Cons: None