Ibanez
Let me start by saying it is unlikely that the amount CX provides will change whichever scheme you choose. I am in MPF - I was a permanent HKG resident when it was introduced so kinda had to. CX puts in $1K for me and $1K from its 15.5% PF contribution...the rest I get as cash in my salary(and unfortunately taxed as salary).
I really can't answer your specific question other than to say I think PF is a lot more flexible in terms of what you invest in and how you get your money back. I firmly believe that CX will still end up paying you 15.5% somehow or another...whether it be MPF+salary or just MPF or just PF.
Also...not supposed to give advice, but if you are reasonable with money don't volunteer of any of your own money as that 5/10% could be used to pay off mortgages somewhere else. If you have no idea or no self discipline, then adding your personal 5/10% is a good idea.