Cathy F - no question.
Better service, better food, better seats, better everything.
A380s are large. Even though you are in F, why fly with more people on an A380 given the larger crowds in the boarding lounge as well as waiting for bags to appear (BA priority tags never work).
When paying for a premium seat, I do not really see the excitement of flying on a bigger plane with more people, unless it offers you better seats or more space in the premium cabin.