So what is the difference between hiring a costume or buying one to keep?
If you want a great quality or elaborate costume or something like a large fur animal hiring can be far more economical. Many really good costumes are simply way too expensive to purchase just to wear once.
If you are a standard size or think you may want to be the same character over and over again buying your own costume is a good idea. Buying also takes away the pressure of having to return something on time or in good order (great if your going on a bucks night or taking something away for a extended holiday).