For me it would be BA FRA-LHR-LAS as it would be J on a 744 (over a 777 on AA). That swings it enough for me.
Your LHR-LAS flight will be crewed by Mixed Fleet, but I have never experienced any of the horrors that some on FT have - I find them extremely friendly, helpful and passionate. I would not avoid a flight on the basis of it being MF operated.