If I'm with others then it's fine, assuming the train isn't busy, to wander down to the buffet. Otherwise if I'm on my own, I don't generally like leaving my seat. If I have to leave my seat, it will more likely be to go to the toilet.
Of course if the train is busy, both options may go out of the window. Trolleys can't get through and I can't get to the buffet.
Overall I think a trolley will do better, and with modern technology you'd think you could have a small area on the train for a trolley to restock and maybe one day have the ability to order to your seat using an app. That helps staff manage demand more effectively, as well as increasing sales as you can order in a more relaxed fashion and also know everything on offer.
I'm sure today a lot of people don't know everything they could buy, or waste a lot of time asking what is available.