AGSF

I believe this is not correct, and that you need status PLUS a oneworld boarding pass to access another OW lounge other than the lounge of the airline you are flying.

But it appears OP has Emerald status, so access to CX's lounge in HKG should not be a problem.

As to the seat, I do not like tight spaces at all, and CX's business seat with those high walls is just too confining for me. I like BA's CW window seats. I find that even (especially) with the screen up I do not feel confined at all.

All things considered, I would take BA. I think the CX new J seats are too claustrophobic, and BA CW (with the exception of the EF seats) are preferable to the herringbone seats on CX.