I did HKG-LHR both on CX and BA in F. I like the CX seats better, because they are wider. Also you are not sleeping halfway in the aisle when you have a middle seat (avoid them on BA). Also (as mentioned before) CX has 12, BA 14 seats in the same space. The BA window seats give you a bit more privacy because of the layout.

The CX lounge is by far nicer than the BA in HKG. Also free 802.11 vs. paid in the BA lounge.

The food on the plane is great on both airlines, but since i like asian food i prefer CX.

The service is better on CX, especially during the night. There is somebody there all the time.

Also CX has a dividing wall between the galley and the seats, which cuts back on noise - especially when in the last row.

I would do both of them again, both are excellent in F. Here is my personal ranking of longhaul F of all the airlines i used in the last 12 month or so:

1. Cathay on 747 (Betsy)
2. Singapore on 747
3. Qantas on 747
4. British on 747
5. Lufthansa on A340
6. American on 767 and 777 (flat seats)
7. Lan Chile on 767
8. Lufthansa on 747
9. United on 747 (suite)
10. Iberia on 747
11. United on 747 (non suite)