A recent article at Bnet states that Twitter is somewhat less valuable for building lasting partnerships with customers than Facebook. However, the evidence provided is somewhat anecdotal. Both Twitter and Facebook are very good tools for business, but they may be best used as a complement to each other, rather than one or the other.
Certain businesses will do very well with Facebook and others with Twitter and many with no social networking strategy at all. The article appropriately points out that it is a significant time committment to manage these social resources, so taking a macro view and deciding where your finite time is most effectively spent is likely the best business decision you can make.