I am also heading out to Hawaii in December and back via San Francisco in January. As a note of caution I personally wouldn't book flights determined by aircraft type, for me I was doing that trip regardless of plane type and booked the flights based upon convenient departure times.

Since booking my flights the LHR - SFO leg started off as a 747 then went to a 787 and is now an A340. The SFO - LHR leg went from a 747 to a 787.

Who knows what I will eventually end up on, but either way I will make the most of whatever I get.

Enjoy Hawaii and California they are without doubt my favourite destinations.