Why don't you go for F class on MH? The difference shouldn't be that much.
Most importantly, where do you want to earn miles from? CX or MH? You may earn Asiamiles if you're booked on CX flight number but on a MH aircraft.

MH has great satay you know... seats are ok. No lie flat beds in Business Class yet.. and meals on MH should beat CX in this instance. MH serves meals a la carte on a trolley for you to pick to choose.

PEN-HKG is too short a route to really compare the two carriers. Maybe you can fly out on MH and back on CX... but both on CX flight numbers, and earn miles on Asiamiles.