Both. Stumble upon bring more traffic, but many people don't stay more than 1 sec on your website. Digg is better for seo but is more difficult to obtain good results with it. However, for a big site with lots of visitors (and the possibility to gain more votes) digg.com is the best solution (huge of traffic and many backlinks).
I opened a pool section for this thread... let's see which of these social bookmarking websites is the winner
I like SU more than Digg because its easier to use. I used to have a PR 3 profile on SU but now I don't use any of them.