I do a mixture of both, partly to save space at the table, and partly because I cook portion sizes for most things (× no. of guests) as I don't want there to be lots of leftovers.
That means I portion out the meat and the roasties, so that everyone gets a fair share. Everything else goes onto the table for people to help themselves to.
For example, I dish out 4 roasties and a portion of meat each first (checking preferences, obviously) then all veggies/extras/leftovers,etc, are put out onto the table for guests to help themselves to, if they so wish.
Serving and cooking wise, I work on 5-6 roasties per person, 150-200g meat per person, 3-4 portions of various veggies pp, 50g stuffing pp, 2 pigs in blankets pp, 2-3 Yorkshire puds each and roughly 200ml pp for gravy.
It seems to work well, and apart from the meat, which is planned to last a few days, we're not left with lots of other leftovers, which is great.