The figures you provided do not look right unless you are talking about 2 different flats.
For a mortgaged flat with monthly payment of HKD30,000, its market value should be around HKD6.6m (assuming advance ratio of 70% and interest rate of P-2.25%). Out of the monthly payment of HKD30,000, HKD18,000 will be interest portion.
For a leased flat with monthly rent of HKD10,000, its market value should be around HKD2.5m (assuming normal yield of 5%).
If you reallly mean to choose between "buying a larger flat" vs "leasing a smaller flat", it all depends on your actual need.