Cadence is a word which has a meaning (musically, a modulation of a sound or voice!) whereas Cadance appears to have no meaning. I'm not sure whether Hasbro chose the name because of the meaning, but in other uses I've seen it certainly appears to be the former.
"So what if my wings can't get me off the ground?"
I already read that thread, Vinyl! But thanks! In short, there's a lot of evidence both ways and there are a lot of arguments both ways. We could have a big all-in-good-fun argument about it here if we wanted to!