Hiya,
I fly the 'bus from LHR, and would recommend you took the 75! Nothing to do with the Airbus v Boeing argument, more a case of having more options in the bag. You will find it easier to convert/transfer to another boeing when the long haul bug bites (I have long since forgotten what trim is! - and I love it
)
Seeing as a larger percentage of the BA fleet is Boeing, I would take the 75 is offered. The 73 LGW aircraft dont have long to go on their leases, so things could change very rapidly in the shorthaul camp.
Well done on getting in!
Frank