First things first. Let's establish the difference between an interior designer and an interior decorator. The short answer is, designers are technical and decorators are aesthetic. Designers are trained in space planning, building codes, form & function, Designers adorn spaces with furnishings and décor. Which one you should hire depends on what you need.
Budgeting wise, interior decorators are oftentimes the more affordable option. They are great in a project which requires sprucing up with new pieces. They offer help in shopping and the overall feel of the space. There is also no formal education requirement for decorators though many of them have taken some sort of training class to enhance their knowledge. Then there are other decorators who just have an incredible eye for pulling cohesive spaces together. They produce some of the most beautiful rooms I've ever seen!
While a few interior designers offer decorating services as well, the most efficient use of a designer is when it's time to get down to the nitty gritty. In contrast to decorators, interior designers do require formal training, whether it be a 2 or 4 year degree plus continuing education. Some areas also require designers to hold a professional certification, similar to passing the bar to become an attorney.
If your project requires the use of an architect or engineer, than most likely you could benefit from an interior designer. They are the first step in the design process because they lay the road map for exactly how everything should go. After all, who wants to spend tens of thousands to hundreds of thousands of dollars blindly? A designer will start with consultation, then concept, then drawings to create a design plan specific to your needs. From there, many will also offer project management services which basically makes them the liaison between you and other tradespeople. Making sure the project stays on schedule saves time and most importantly, money!
Either option is a great choice for the right person. The services of both interior decorators and interior designers should be considered an investment. These are people who eat, drink and sleep details. While fees vary from one company to another, the fact of the matter is those fees often don't compensate for what they give. They invest so much of their personal time & energy to help change the lives of those who care about the spaces they occupy.
My advice to you in choosing between an interior designer and interior decorator and which to hire would be this. Evaluate what you want, your realistic budget and desired timeline then research and consult with at least three professionals. Choose the one who most fits your expectations, design style and/or personality. If you use these simple starting points, then your experience should be great and your dream space will be a reality.