I have both and like neither
Twitter is good if you want to see what the famous pretty people are saying whilst knowing that you'll see major news before anyone else.
Facebook is a nightmare really. Perhaps if it was possible to segment people into different groups.. work/friends/family or something. Even then though it'd be full of invitations to silly games and people tryin' to sound clever ;-) Problem is it's now the one place you can almost guarantee to get in contact with someone.