Often, the shorter word carries more "punch" than the longer word. That is certainly true in this case. Both words are four syllables long, but ferocity is shorter in both speech and writing. Especially since you are using the word to convey the sense of strength or intensity or power, the shorter word is preferable. I wouldn't use ferociousness at all.Could you explain the difference between ferocity and ferociousness?
I hope that helps.