The stability ball makes this movement ten times better and offers a much better progression and regression.
Take a look at the video. You'll note that the range of motion goes from extreme deep neck flexion all the way into full extension. The key is that the neck is loaded in every position, as opposed to just the end position of a normal bridge from the floor.
The video also shows a slick regression if you don't have the neck strength for the more difficult variation, as well as a neck stability drill that can be great for fixing head-forward posture.
Another great part about neck bridging is that it helps you coordinate the entire posterior chain to function correctly, as shown in the video where the glutes and spinal erectors hold up the bridge.
If you've already been doing neck bridges on the floor, the stability ball neck bridge will unlock a new level of neck strength and, accordingly, a new level of overall body strength.