Your journal is an organic personal development tool. You establish a relationship with it. If it is working for you, it will fit your personality and beliefs. If not, it will hinder your writing. For example, if you are a vegetarian you may feel uncomfortable writing in a leather bound journal. If you are naturally frugal, an expensive journal may impede your ability to write freely for fear of making mistakes. Asses your preferences before you venture out to make your purchase. This will keep you from being overwhelmed by the large variety of choices that will confront you, and it will keep you from buying on impulse.
Functionality
You need a journal that will fit your lifestyle. If you are on the go most of the time a small journal is a good choice because it will fit easily into a bag pocket. If you choose a small journal make sure that it will remain open on its own when you lay it in your lap or on a desk. A spiral bound journal may be the best choice of binding for a small journal because you can fold it back and easily lay it on your lap if you find yourself stuffed into a coach seat of an airplane, or on a long car trip.
