If you believe the Alexa graph, Digg is a significantly more popular site than Reddit, with the gap growing larger every day.
I find these results surprizing. I've had half-a-dozen entries on the front page of Reddit, with each yielding from 2500-5600 distinct referrals per day. In comparison, I've had a front page entry on Digg for a day, only bringing in ~750 referrals.
Of course, a single entry isn't a very good sample, and it's entirely possible that most people just weren't interested in the link -- that it was a fluke that it got on the front page in the first place -- but I've seen several other stat watchers mention very similar stats (that a front-page of Digg yielded them 800 or so visitors per day), so I'm not basing this comment simply on my own observation. I've looked for exceptions to this, and found one individual who had a broad-interest link atop Digg's front page for 24 hours straight, and they claim that for that day they received a total of 7200 distinct referrals, and then it rapidly tailed off, disappearing in two days. That case seems to be the exception.
One possibility is that Digg offers more link diversity and thus the much greater traffic is dispersed, significantly reducing the impact on any one link. Alternately, perhaps Digg users spend more of their time within the Digg community, rather than following the links (in the same way that many Slashdot readers just make assumptions about the linked article, responding accordingly, rather than RTFA).
Another possibility is that Digg caters to a crowd that is more likely to have the Alexa/A9 toolbars installed, both of which feed back the stats that are used to drive the Alexa popularity metrics. Given that they're somewhat infrequently used toolbars, and are much more likely among certain crowds (and seems to appear in clusters), the traffic rankings are a bit of a crapshoot outside of the top sites -- Here on yafla I've had days with 6000 visitors where my Alexa ranking doesn't budge, whereas other days 2000 visitors cause it to quintuple.