I think if you'll be based in London with a fair amount of transatlantic travel, CX's program would be a good option.. My reasons:
1) CX let's you credit both BA and AA. Both BA and AA won't let you credit the other airline's ticket for trans-atlantic. This leaves you with more options out of LON.
2) If you'll be flying some economy or premier economy, again, BA does not give tier points for most economy tickets, and only 25% award miles. Cathay is far more generous with crediting economy miles.
The one disadv that CX has is that it is much harder to reach elite level tiers, but once you do, it's certainly more exclusive with greater benefits than other OW Elite programs.