Digital speedos average the speed over the last (eg) second and when it changes will usually do some form of animation between digits. This way you don’t get constantly fluctuating numbers.

As for analogue versus digital. It depends how you think of a speedo. If you think of it as a percentage of maximum then analogue is best (like a fuel gauge) but if you think of it as needing to know your ‘exact’ speed (like temperature) then digital is best.