A lovely sit-down dinner may be just what you are after for a sophisticated and elegant Christmas party.
A sit-down dinner however can be a little more time consuming and labour intensive to organise due to the need for full table and place settings, seating plans, table numbers etc.
The menu choices can also be somewhat limited. A formal set menu generally involves selecting two different options for each course, which get randomly placed around each table. This can be absolutely fine for many guests however may not please others. It can be tricky to allow for any special dietary requirements, as these need to be planned for in advance.
However having a formal sit-down meal also means your guests get a much more substantial meal and can sit and eat in comfort (without the need for being on their feet all night). It also allows for a more structured evening with plenty of opportunity for speeches and any other formalities you wish to include.