The V551 has far more features, such as bluetooth (for wireless
shortrange connections to either pc's or headsets), the v551 also has
video camera capabilities, it is a worldphone, not sure if the v400 is
also a worldphone (meaning it will work in global
gsm networks).. if
you want to compare them go to
www.phonescoop.com and compare.
Though for the 20$ I would defenitely go for the V551 - you may find
later you regret it when you might have actually needed the bluetooth
or wanted the video capabilities.