On Community Migration from Digg to Reddit
kqed.org
kqed.org
I looked up the metric you used, and compete.com describes it this way:
"Use Velocity to understand how much more or less time people spent on the site, or their level of engagement. If a site can garner and sustain more of an individual's time it should generally be considered a good thing."
So, it seems that reddit visitors spend more time there than digg users do on digg.com.