This is a tough one. I prefer the 747 over the 777 (talking BA here) because I love the upper deck privacy in Club World and hate the flying youth hostel feel of the 777 in J. Also the First cabin on 747 (in the nose) is much preferable to the 777 which has a galley at the front of F. While this preference wouldn't get me to fly to another city to avoid the aircraft I may fly at a different time of day in order to get the preferred choice of a/c on the route.

On airliners.net there is a lot of talk about flying on another airline (with inferior service) to try out an A340-500 as opposed to an A340-600 or the like. In that instance I couldn't give two hoots about the a/c type.