The truth is that there actually is a significant difference between a nutritionist and dietitian, although they are often used interchangeably.
A nutritionist is defined as specialist in nutrition. Depending on state law, however, a person using the title may/or may not be trained in the science of nutrition. There are certification programs, but no licensing is available.
Where as a dietitian is well-educated in the science of nutrition, required to pass a national exam, and must receive continuing education in order to remain registered. Membership in the American Dietetic Assoc. and state licensure are optional, but may be required by an employer.
Don't be afraid to ask about the qualifications and education of nutrition professionals from which you get advice.