Digg vs. StumbleUpon
There are many Web 2.0 sites vying for the attention of users. Two related websites that I use frequently are Digg and StumbleUpon. Both do a good job of introducing new sites to me that I may otherwise have not discovered. However, I'm finding that I'm slowly leaning toward StumbleUpon as a more effective tool.
What I Like About Digg
Digg's voting system has been the topic of many discussions, but I'm going to ignore the accusations of unfair manipulation. Instead, I prefer to focus the utility of having a vote count associated with every submission. That is, such a count makes it easy to establish some threshold for filtering what I will read and what I will ignore. An easy to apply filter is important to me because there's just too much on Digg to keep up with.
What I Like About StumbleUpon
Like Digg, StumbleUpon uses votes as part of its algorithm on what to show. However, rather than being required to scroll through a list of items to find something new, StumbleUpon's interface takes me right to the website. I like this approach because I get to evaluate a site based on the owner's description and design.
I also like the voting and submission mechanism. Both can be done while on the website of interest. My movement from website to website isn't hindered by having to constantly return to a central repository like Digg.
What I Don't Like About Digg
Title and description manipulation is rampant on Digg. Sure, some people call it being creative, but in many cases the top stories become popular based purely on a sensationalist title. In addition, the text-based nature stifles any creativity that may exist on the destination site. Some sites just need to be seen to be appreciated.
If you want to be an active voter, you need to keep two windows open. One for the Digg page and one for the destination site. After assessing the site, you need to remember to go back to Digg to submit your vote.
What I Don't Like About StumbleUpon
My biggest complaint with StumbleUpon is that it accepts paid submissions. This is great from a publisher's perspective (I've tried the service myself) and it's also a good way for StumbleUpon to defray it's costs. However, I feel the quality of sites in rotation is decreasing as more and more are artificially inserted in to the queue. Sometimes I get the impression that several sites in a row have paid for their inclusion which can be frustrating.
So neither system is perfect. But regardless of current quality concerns, my preference of being taken directly to the website ala StumbleUpon makes it the winner in my book.

Digg is garbage compared to Reddit. Stumble is different
Stumbleupon is great for slow building traffic. Digg gives you bursts then dies. So its best to use digg to get links and su for traffic.
Sarakastic,
I guess different social networks attract people with different dispositions. The folks on Digg seem to be ornery. At least a percentage that are very vocal seem to be.
To me, stumbleupon is more fun, & the community in general just seems happier