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 rectangular tables are the most common, which shows that they are a practical, functional and comfortable option. They are perfect for elongated dining rooms and they allow for more diners in less space. In a small room, a rectangular table can be shoved against the wall and still be perfectly practical.