I prefer digg...its more straight to the point with its titles and gets you quality traffic (plus way more than SU) since its not so much a discovery of articles by surprise but a catchy title that attracts users interested in that headline.
the best part about submitting to reddit, is if you have a legitimately intersting article and its not spammy crap, you will get VERY good traffic and legitimate rep, which means you can continue submitting articles and people will recognize. but the moment it looks spammy (aka people who blog youtube videos) the community will basically shut you down. nothing stopping you from creating a new account tho.
let me give you just one more piece of advice, try creating a website like this: http://thehomebased.com/ It comes up many times on reddit and sometimes on digg, it looks like they are posting real content but a lot of the site has affiliate links to amazon etc. create a blog like that, with links to your affiliate url (not super obvious though) and start submitting it to all the sites and you should get some decent traffic. make sure videos and/or pics stay interesting tho.
If you get on the front page of DIGG, your traffic will explode for about a week. This can happen with stumbleupon too, just not as much.
In my personal opinion I would focus on Stumbleupon because it is not as social network oriented, which means you can come into the game sort of late (which it is) and still make an impact. Join groups within Stumbleupon, add your websites (but also add websites you like but aren't yours, such as DP) and add the appropriate tags for your pages. There are days when I have received hundreds of Stumbleupon traffic but not much from Digg. You either get mass traffic from digg homepage or get none from what it looks like...the site is an overall bookmarking site with no focus so you have to go big or go home on their network