One of the first things you will have to ask yourself is where you will be placing your dining table. A round table curiously seems to draw all elements that surround it towards it, thereby anchoring itself (and the chairs) visually more than a rectangular table would. It also takes up less space.
The space that your dining table inhabits should have a natural impact on the chairs you choose. For example, if you have a tall table in a contemporary kitchen area, you might prefer some tall bar stools, to give a breakfast bar look. Whereas, if your table is in an open-plan room, you'll want chairs that align with your sofa choice and other decorative touches!