Premium does not always mean that a theme is better. I have used hundred of themes, both free and premium, and have found that many of the good free themes are better than a lot of premium themes. So, premium just mean it's a paid theme usually and may or may not be better.
As to the main question by the OP, it will all depend on how much customization you will need that will contribute to if you need to get just a good theme or hire a developer. If you want something that has extra functions, looks, elements that can't be found in a free or premium theme, then a developer would be able to provide that. Otherwise, save the money you would pay a developer and just get a good premium or free theme and tweak it to fit your needs...