Credit cards always come interest-free until at least two weeks after the next statement date. And if you have the money anyway then you just set up an autopay to pay off the full amount every month. That way you never incur interest charges. Using credit cards in this way is better than using cash because many places have special offers for purchases on certain brands of credit cards, and you can earn AsiaMiles or similar on your card.
Savings account rates in HKD are negligible.
Home loan interest payments are tax deductible up to a limit of HK$100,000 per year (i.e. you can deduct that amount from the income on which tax is assessed, not that much tax). See here:
http://www.ird.gov.hk/eng/tax/ind_hli.htm
Rental payments can also be used to lower the tax bill, but it requires the co-operation of your employer to structure your compensation so that some of it is defined as "Rental Allowance" and your employer will need to validate the rent that you actually pay and make returns to the tax office as well as you. Most larger employers are familiar with this system. It is not a simple deduction from taxable income. Details are here:
http://www.ird.gov.hk/eng/pdf/pam44e.pdf
In simple cases you don't pay tax on Rental Allowance that is in excess of 10% of your other income.
Which would be better for you requires a calculation involiving your income, the rent you intend to pay, how much of your income is declared as a Rental allowance, the price you might pay for a property, and the interest rate on the mortgage.
All residents of HK are subjct to the same tax rules, expat or not.
Coming to HK 
