If you have one formal and one casual dining space, you can really go to town with different chair designs for both. A casual spot will do best with sociable, comfy seating that naturally leads to conversation, whereas a formal setting is where you can play around with luxury fabrics. If you only have one dining room, you're going to need to combine all of these elements in one set of chairs, if possible!
As mentioned, rectangular tables present a certain advantage in terms of capacity that round- and square ones do not. Imagine having to place all the diners to be seated at this elongated table at a round one – you’d have to have a pretty big round table!. For big dinner parties, it’s best to opt for an elongated/rectangular table, as you get more space for chairs and guests.