Hi Adapsys, welcome
I wouldn't worry too much -- it's not unknown for equipment to switch, so you might end up on an A346 on VS8 in any case. Also, I believe it's not unheard of for one of the LAX flights to be cancelled, with passengers switched to the other one, if loads are very light.
As honey lamb says, you may be thinking of the Gatwick jumbos, which get a bit of stick. (Although I think some of those aircraft were fitted with UC more recently than the LHR planes.)
Personally I much prefer the Airbuses because I find them so much quieter than the 747s. You didn't say which class you are flying -- but if it's UC then even I wouldn't mind the 747 too much. I think the bigger differences are to be found in economy and premium economy.
In any case, given the choice I think that the type of plane is actually a secondary factor. I'd decide based on which departure/arrival time is better for you. If you are firmly on Pacific time, then VS24 gives you a much better shot at some sleep.
Have a good trip.