From my personal experience, the only deciding factor is the time. I am into sales and got to attend lots of meetings. At times, you will be running short of time and yet hungry. Then I would love to prefer buffet as decisions and servings can be done quickly.
But on the other hand, during a relaxing evening, with friends and family, I always pick a-la-carte and lots of dicussions are made on the choice of food and then once ordered, goes back to our favorite topics, listens to live band/music etc.
So according to me, it is the time which makes my choice between buffet and a-la-carte.