WOW you must have some serious cash or a very important job for that amount of premium class travel , yes 18hrs is a very long time even if you are Airline/AV Geek, If it was me i would choose Emirates personally as i am a big fan of Emirates and plus i am a gold card holder and that trip would defiantly boost my miles and tier status also the shower onboard does help :P. Its is a shame you not departing in March as BA are switching the 747 to A380 on the LHR-SFO route. Im sure which ever airline you choose you will enjoy , it is a lot better then being in economy ......