I think My Beau loves Nunn Finer xD
It honestly does not matter. I got my breastplate at a consignment shop for ten dollars and it was in pretty darn good condition. Most breastplates are very adjustable.
So what I suggest is taking a horsey friend who knows about breastplates to a consignment store and buy one, take it home and adjust it to your horse with your horsey friend helping you adjust it right and remember exactly how they work it. Then just use it every time you ride.
I can't ride without my breastplate because one horse has big withers and is round and the other one is just round so I slide