So much depends on the layout of the room as what is the best choice for the space.

i think the main downside of a round table is that when you are 4 people sitting at a round table meant for, say 8, its rather lonely.... like being seated at the wrong size table at a restaurant. at least at a rectangular table you can sit together at one end. also, harder to pass the food across, longer way to talk across.

if the table is small it doesnt matter, of course.