Buffet all the way.
There's a wider selection, proper milk in the tea, a social area to stand if the train is busy, the option to have a drink with friends on the commute home and you always know where it is.
Trolleys block the aisle, serve less, take ages to move through the train and can be removed from the service all-together far easier than a buffet. Not to mention the inability to be served at a time of your chosing (if at all!) if the attendant insist on serving only at the seat.
Trolleys are undoubtedly better for people with impaired mobility, but in all other regards they seem pretty flawed to me. Of course, they both pale in comparison to the hospitality afforded by a proper restaurant carriage!