This is a NO Brainer!

CX definitely, unless you have certain urge to torture yourself in LHR! (also QF leaves from T4 which is a dump now - not T5 BA's hub)

QF's F Class is almost 10 years old, it is much narrow and cramp compare to CX's new and old F Class. Service IMO is better on CX (including better amenity kits and PJs). The cabin is much better (with a much nicer bathroom - although the QF one is not bad - with a window, well CX has 2 windows)

Really take CX and you won't regret it. Also stick with the Chinese menu as I heard the F Class Western choice is a bit average.

Also I assume you get more time in AMS as well with CX's timing?