There is no right answer to the question variable or fixed. It depends on many factors. The most important factors are:
How long will you be here in HKG?
How certain are you of your previous answer?
Are you likely to upgrade your housing in the future?
For the guy who says 5 years then out, then fix. You have just taken a variable out of the equation - the only variables left are HKG interest rates and house prices;-/
If you think you will pay this place off in say 5 years then upgrade, then I would say float as chances are you will be buying a bigger place. You will have rent of the first place to assist in buying second place whether rents have gone up or not. If you had fixed 4 years ago you would now be trying to buy a house that has doubled in 4 years but your assistance has not.
Almost half of our pilots have joined only knowing a rising market. There are some guys who bought in 96 or 97 who are still waiting for prices to rise to what they paid back then. They saw equity losses of over 50%!
For the majority - probably staying for long term but the rates are attractive now.... Just imagine three scenarios;
1) house prices/rents don't change - then it really didn't make much difference fixing or variable.
2) house prices/rents fall - whilst you will get less money to pay off your house you will find buying the new place more affordable as if rents are falling presumably the purchase price has fallen too. Rents are 50-70K a month - prices are 5-10million. It takes a lot of years of a higher rent to wash out the loss on a property!
3) prices/rents rise. You will be able to pay off current place slightly more quickly but more importantly you will have a bigger monthly allowance to buy/upgrade to your new property.
Worst case, massive fall in rents/purchase price - you have 2 years before your assistance is reduced - then that is locked in for 2 more years etc- so a very slow (hopefully) decline in your assistance.
If you are sure you only want to buy one place and then are happy to rent after that, then fix-you have no risk to your repayments and then once place is paid off you will have the CX allowance, whether it has gone up or down, to rent a place.
Simplistic advice - when in doubt, float. If you are certain you are leaving in 5 years, fixed. The breakeven between the two in a rising market is 4-8 years depending on the rate of increase.
Try to buy for an amount such that it will be paid off in 5 or so years - 5 years is a long time in CX and HKG! Plans change quickly- you dont' want to have to forgo a basing opportunity as you need housing assistance to pay off a mortgage that is worth more than the property!
My other advice, take any financial advice a pilot gives you with a grain of salt;-)