It totally depends on your niche and existing traffic too. I do well with stumbleupon and Reddit, digg doesn't drive much traffic my way. However it is best to bookmark your pages to multiple sites at once to increase the probability of getting higher and closer to their homepages.
Both digg and stumbleupon are good for traffic generation. But for traffic you can also try doing promotion on social media sites like facebook and micro blogging sites like twitter which will help you a lot in generating more traffic.
I will go with stumbleupon because there the public are fun loving and loves to visit new sites. Stumbleupon traffic is anytime better than digg traffic because digg is the site which was actually created to criticize any new websites, so it will give you traffic with more of critics. Now you decide, you want critics to your website or fun loving public?
Yes, I strongly agree with you because after bookmark any page of Social Bookmarking Site thereafter you have to need broadcast your post to Social Media Sites like Facebook, Twitter etc. It will be very helpful for generating traffic on your site.