So just what is the difference between a Corset and a Bustier? The words seem to be used interchangeably in advertising.
Corsets are formfitting, extending from under the arms to just above the hips and usually made of flexible materials like cloth, vinyl or leather, are stiffened with boning (aka ribs or stays) and laced. They are worn to enhance and/or emphasize a curvy figure by slimming the waist and accentuating the breast and hips. Historically a corset was worn over a Chemise which back in the day was more shift (made of cotton or linen) than the sexy figure skimming gown that is the modern equivalent we associate with the word today. (There are also corsets that are designed to present a tube or straight up and down silhouette and an entire fetish community for wearing and collecting corsets - corsetry.)
In comparison, a Bustier is a bra with an elongated torso that lifts and enhances the cleavage. In fact the word bustier is from the French word for bust. A bustier is usually advertised as any formfitting sleeveless and often strapless top, worn as lingerie, an outer garment and/or evening attire.
So while one is designed to enhance both the bust and hips, the second is primarily designed to enhance the bust.
Only you know what you are in the mood to do tonight.