twitter is good if you're the type to update things (ie. short "blogs" of just a few words) all day long. Facebook has way more options, features, things available, and people involved. Twitter is good to follow things you are interested in - for example, most every band has a twitter account and post news llike concert dates, albums, etc. Facebook does all that too, however.
There is nothing you can do on Twitter that you can't do on facebook. There are alot of things on Facebook you can not do on Twitter. Just choose which format suits you best.