Firstly I assume you are talking about joining CX and you are trying to choose between Mandatory Provident Fund and the CX sponsored Fidelity Provident Fund.
Not giving financial advice as to what to invest in....but...
PF has lots of choices in terms of different country funds/cash funds etc. MPF I think has a much more limited range...I think only 3 or so...like balanced/growth/conservative etc.
PF can be accessed any time you leave for a basing(you pay HKG tax on the unvested portion...if you have say $100K in the PF, and you have done 4 years, 40% of the PF will come untaxed and you will receive the other 60% taxed at HKG rate..currently 16%). At the moment normal retirement age is 55...so if you stayed employed in HKG until then you will then be able to leave HKG with 100% of your PF.
MPF is a concept meant for local citizens of HKG rather than for expats. You can only get it at age 60 or earlier if you leave HKG permanently. It would be difficult to prove you have left permanently if you go on a base.
The returns of either scheme will depend on asset classes chosen.
You can change where the monthly PF contributions go on a monthly basis, as well as moving past contributions. EG if you decided you had a good run with equities and wanted to switch into cash you can choose one of the many currency cash funds.
If it was me...I would choose PF...but as they say, seek your own guidance...blah blah blah