On one side there are high PR social bookmarking websites, like http://www.digg.com and http://www.reddit.com/ that use the "nofollow" HTML tag and on the other side are low PR websites that don't use this tag and offer quality links like http://business-planet.net/ , http://www.pricespecs.com and others. Which of them do you consider that offer more for a blog?
High PR sites are likely to give you more traffic. But low dofollow will pass PR so I suggest using both in moderation
Try both. For a good list: http://mediapress.co.za/social-media/dofollow-and-nofollow-social-bookmarking-list/
A good combination will result in good results. Try both of them because every link to your site is a backlink.