There is no difference in the qualification. Cosmetic dentistry is a marketing term that generally refers to dentistry that is seen in the front of the month. Any general, family dentist can call and market themselves as a cosmetic dentist. There is actually a specialist in cosmetic, restorative and and implant dentistry and it is called a prosthodontist.A prosthodontist has to be admitted and complete a post-graduate program after dental school.
As far as teeth whitening goes, any dentist will do just fine.