I really think both BR and CX offer a decent premium economy class product with similar seats and seat pitch and even same luggage allowance (2 pc of 23kg/55lbs), and I will base my decision on:
1. Price - go with the cheaper one
2. Planes - EVA Air - Elite class all the way from BKK to LAX and back - CX has this plane lottery on flights between HKG and BKK
3. Mileage - are you an Oneworld (CX) or Star Alliance person (BR)?
4. Time - Going via HKG or TPE has similar flight time, and TPE is a bit smaller and easier for connections, but HKG is a nicer airport with more things to do in between. Go with the airline that gives you the best schedule
Both are good options!
Carfield