Ive flown 747, 787 and 340 in the last 4 months with Virgin and the 787 was the best as it was the newest plane with better in flight entertainment systems and I like the bigger windows. I didn't fly economy but did have a stroll down back an it looked better than the 747/340 economy so if the 787 is an option I would take that. The 747's they used on the Heathrow/JFK route are very tired. I have flown upper from JFK with Virgin and even that feels tired with areas that frankly need a good clean. If you can get into Premium Economy I feel it's worth it as the extra leg room and comfier seat are worth having as is the check in and rapid boarding compared to economy especially on a busy flight. If you are in a 747 in economy and are able to book early then aim for the upstairs economy section as the 6 seats at the front (only about 6 rows in total) that have about 10 feet of leg room, though they are a little tight due to having tables in the arms.