I just kinda assumed it was like Savvy said. I always associate a breastplate with English riding and a breast collar with western riding. Hm, it may be another one of those things that changes depending on where you live and what group you grew up with.
I've used both interchangeably...sometimes it's easier for a person to 'get' what the strap is, if you say breastplate; I get more "what is that?" using breast collar...they assume it's a literal collar, hahaha!