We go to lots of both types of events, and honestly the quality of the food is the same -- it depends on the caterer and menu much, much more than on whether it's buffet or plated. Both can be great, and both can flop.
The formality level difference is much greater than any quality difference.
As to practicality, you really need to consider the geography of the room, the number of guests and buffet tables (for buffet) or servers (for plated). I've been to buffet-style events that were completely ruined because the buffet tables were too few and too poorly placed to be replenished without waiters cutting through guests. In my experience, buffet works better for nibble menus or stations with lots of choices, but for a "regular" meal, plated or family style works at least as well, and plated is more formal.
The type of event really does matter. I don't mind standing in line with my friends or cousins (like at a wedding) as much as I would at a charity event where I might not know anyone. And if I'm in an evening dress and heels, nibbles would be fine, but I won't much appreciate serving myself a dinner plate full of food and carrying it around.
All that said, buffet, family style, and plated all work, for both formal and informal events. There's no one-size-fits-all "best" -- too many variables: type of event, guest list, type of food, number of guests, details of venue, and much more. Yet another example of something not to decide in the abstract. Consider all those factors, and discuss it with the caterer , and you will make a good choice.