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.
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.