They say Digg is bigger than Slashdot than these days. Bigger, better, newer, 2.0 and what not.
Indeed, I find myself scanning the Digg front page more often than Slashdot's ditto. Far too often, actually. Obsessively often? Well well, I actually get proper work done now and then.
Still, it's simply not true that the quality of the "news" on Digg's frontpage is higher than the news on Slashdot. On the contrary, Digg suffers from a disgusting amount of mob mentality. Half of what's on the front page is not news at all, merely short text snippets propagating rumours that everybody's already heard or views that most people already agree with. Which is why they get digged to the front page: people like to hear/read what they already believe in.
And this is not really a problem with Digg, it's a problem with people. (And like all problems we can't really do anything about, it's not even worth thinking of it as a problem, just as a fact.)
Slashdot, on the other hand, has sometimes-competent editors that sometimes put an effort into selecting and editing stories. While I might learn something I didn't already know from either Digg or Slashdot, I'm far more likely to learn something new about something I didn't already know anything about from Slashdot. Crucial difference.
So, back to the question: Why do I keep reading Digg when Slashdot is better? Because the Digg is updated more often, and the items are shorter. It's that simple. I think.