"Out of many, one"
This motto is a clear definition of what the United States of America is. We are a melting pot of different cultures, races, religions, states, and traditions. Out of all of these differences we arise as one people and one nation. This is the perfect motto to represent the United States in a way that includes everyone.
"In God We Trust"
This motto, on the other hand, is an affront to anyone that is not christian. According to a 2008 poll by the federal census bureau that means almost 20% of the nation's population is either not christian, or non-religious. To have a specifically Christian US motto, is unconstitutional. It isn't the will of the people and it doesn't properly represent all the people.