The Servo library's PWM is a completely different frequency to analogWrite's frequency - there's almost a 1:10 ratio difference, and the chances are your ESC won't like a 500Hz PWM signal.
Even if it does, your resolution will be much worse than with the Servo library.
Can you say, hand on heart, that the values you are writing are not the source of the jitter? (I only ask that because it is impossible to tell from the snippet of code you posted)