The difference between these two massacres lies in the nature of those who died. As other editors above establish, both massacres did result in the death of masses of individuals. How they differ lies in the fact that the Sand Creek Massacre resulted in the death of people such as women and children who were not involved in the war directly.
The Fetterman Massacre was an ambush and the total annihilation of a US Army force during Red Cloud's War. Sand Creek was the massacre of unarmed Indian people, including women, children, and elderly, by a US cavalry force. Both involved enormous loss of life, but the victims of the Fetterman Massacre (or the Battle of the Hundred Slain, as Indian people called it) were at least combatants.