Digital speedo is the way to go - just so you know what speed you're taking through the corners and down the straight-aways. I find it extremely useful, as I know my own terminal speed through a corner, and I can measure whether I'm pushing the car to its limits.
Having digital speedo is also useful when watching others, to compare the exact speed they are taking through the corners. However, it is not always all about speed, you can be slower but with a better line and get a better lap time.