I'll fly Virgin's 787 over any of their a340's or soon to be outdated 747s, simple because of the WiFi on the 787-9.

I'm based out of San Francisco and make the SFO-LHR route 15 times a year, and I cannot wait for the 787 in October to take over. I've always opted for the 747 over the a340 as it's the earlier flight, and it seems more comfortable to me.

I'd suggest booking on the 787, it's a cleaner, quieter and easier than the a340. It's really the same product in Y, but I'd still book the 787 any day of the week. At the end of the day, the seats seem to be the same level of comfort on both, the seating configuration may be an advantage to a couple traveling and wishing for a little more privacy.

Also, not sure when you're off to the States, but October/November seem to be a low capacity season for Virgin out of SFO, and you may get away with booking an aisle and window seat, and potentially having the middle left free for your flight?