Softboxes give you more control over the direction of the light and are more efficient than umbrellas, but I'm put off by their cost and the extra gizmos needed to attach them to a flash.
I haven't bought one yet, but from everything I've heard, the Softliter II is a great alternative... kind of a cross-over product. It attaches to a flash just like an umbrella, keeps light from spilling out of the back, and is pretty efficient in terms of light loss. Here's the picture and link with description from Adorama.
http://www.adorama.com/PTSL54.html?searchinfo=photek%20softliter%20ii&ite m_no=4