Interior designers provide a wide range of services. Interior decorator provides aesthetics only like furniture layout, help choosing furnishings like curtains, paint colour, decorative elements, etc. Just superficial changes to a space, really. Most people hire interior decorators when they want to redecorate their homes to spice things up or for a change without knocking any walls down or other such reno work. Interior decorating falls under interior design in general.

Nobody calls themselves interior decorators here in Malaysia because the market is so limited for such a specific service. But a lot of people call themselves interior designers when they shouldn't do so.

You can specify what kind of responsibilities (big or small) the interior designer will be given. So you can say you need redecorating and he/she can do the design work and collaborate with you on the aesthetics, even help you source items like furniture, light fixtures, etc.

I suggest comparing the values of each quotation. If, like the other thread has posted before, you can get more bang for the same amount of buck, go for the one who provides more services.