I'm done with digg... my interests are too specific. Plus after a good two month run at it, I've found digg is more about playing the "Digg Game" than finding good content. Great though if your employer just wants as many hits as possible.
I think they are both great social networks but SU is my favorite. Not too mention my damn Digg toolbar ruined the World Series of Poker for me. I had been patiently watching the WSOP anticipating the final table and three days before the final table aired my Digg tool bar notified me that Eastwood won x million dollars.