Bank loan or HDB loan

Depends on your cash flow situation and the amount of CPF-OA monies.

If you are able to handle the 5% cash and 20% CPF-OA downpayment for a 75% LTV, it is better to go for bank loan. The interests saved can be more than $4k per year, which you can use to invest to further grow the amount.

In the event that mortgage interest rates rise above HDB loan rates, you can try to reprice/refinance your mortgage loan after lock-in period, use CPF-OA to pay for monthly repayment (CPF-OA rates can increase beyond 2.5% floor rate as it is pegged to 3-month local bank deposite rates), or even redeem your invested interest savings in case of difficulty in your regular cash flow.