However digg has more potential because twitter you're essentially mostly limited to your followers unless someone searches you up. Where as digg is very widely known with a good user base. So if you do manage to get on the front page you can get a good amount of traffic!
Another thing I like about Digg is what I found out when I was offering my social bookmarking services. That if you use the specific link when requesting people to digg your site they actually visit your site as well.
So if you post that link when people click on it is considered as traffic for your site as well they can digg your site.
Personally I prefer digg better because of it's potential audience you have.
Also it builds a good reputation if you know how to get diggs.