I use both of them.. both are beneficial depending on how you will do the strategy.. but if I have to choose only one, I'll definitely go with Facebook..
I am a fan of Facebook mainly because it is a better way for me to keep in touch with family and friends, twitter is nice but it is very difficult for me to get my parents to use any kind of Internet service at least with facebook they know what it is..
I use Facebook for personal use but for my business I have a Facebook page I then link to Twitter so announcements are made on both sites.