I have flown both routes several times and would choose LHR as it is a 747 so you have the chance of snagging the upper deck (or pay and get a seat up there) which is preferable (in the opinion than most) to a 777.

I also prefer T5 both in terms of lounge choice / quality (try the B gate lounge if the flight is out of B or C - and it will most likely be) and shopping etc.

I have never had an issue finding a CW seat using miles on either route.

Enjoy your trip whichever route you choose.