If you haven't got spare room, setting up an entirely separate table or sideboard for a buffet may not be an option. Sit down dining may be the best option, spatially.
There is an air of formality to a sit down dinner. The pomp and circumstance of such an event can really add to the specialness of an occasion. Even though I noted that our aforementioned buffet party still retained a lovely formality, sit down dinners can be more solemn and intimate events---which many people prefer for the holidays.
At a sit down dinner, although food will take a moment to pass completely around a table, everyone begins serving at the same time. A buffet requires a line, or at least a modicum of "taking ones turn" that tends to be a little less hospitable.
When going for seconds at a buffet, it is much more obvious than at a sit down dinner! Never mind thirds or fourths...
Folks will have chosen their seats at a sit down dinner prior to holding a plate full of food. Unless the seats are assigned, of course. This prevents the search for your place at the table with your dinner in your hands!