CX!
CX!
CX!
CX!
CX!
Mmmm....after careful consideration, I think I would choose Cathay.

Seat 2A is my second home at this point, and a heck of a lot nicer. But doing both isn't too bad of an idea. Nice to get a feel for each, as they are pretty different. BA is more "homey", but CX is more personal. Though I didn't fly BA to HKG, so I don't know if they Cathay-ize to meet the wants of the pax that will be on the route. Sort of like how people talk about UA having a very different product on their trans-pacifics than their trans-atlantics.
Either way, it should prove to be a heck of a trip.