Having Jitter off will give you higher quality at reduced render time *but* can cause banding like this. In an animation it's much less noticeable. Turning it back one will get rid of the banding, but without increasing samples, in this case you may have slightly more noisy shadows (have to see). For a still we'd generally recommend keeping Jitter enabled, unless there is a lot of vegetation on the ground breaking up the shadows, in which case you won't see the banding anyway. For animation Jitter can often be disabled, giving you less noisy results at lower render times, while still having most of the benefits of Soft Shadows.
Anyway, it's definitely a better-looking result with Soft Shadows!
- Oshyan