If you are promoting a blog post, should one focus on digg or reddit ranking?
Which is preferable and why?
Reddit is better than Digg.
Digg has one front page, and only "power users" ever get anything on the front page. If it doesn't make the front page, you will get very few hits. Reddit lets users decide which topics they are interested in. Each topic has it's own front page. Each front page has it's own threshold of upvotes on an article to get it to that front page. Also, the users on reddit tend to look at more not-yet-front-page posts, and tend to be more likely to upvote you.
I've had my LeanStartups.com blog for two and half years (at least) and TheOperationsGuy.com for about six months. Initially I experimented with Digg for the LeanStartups.com blog, but found that Twitter and Facebook were bringing a lot more quality traffic.
I also found that well thought out responses to articles on Harvard Business Review brought in very solid 25% of my traffic (not bad for 1-2hour per week investment).
Agreed that generally Twitter > Reddit > Digg, but it depends on the blog content.
Often news and entertainment posts do better on Digg than, say, posts about business, marketing, or technology.
Still it's hard to imagine Twitter not being a useful platform for sharing links.
Reddit vs Digg, it depends what visitors do you expect. For me Reddit gives me more traffic. Also check the "no follow" policy for links in Digg.