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!
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!