What is the ruling on borrowing from a usurious bank (Housing Bank) with a diminishing interest rate to buy a house, if it is easier than borrowing from an Islamic bank?

If both banks lend with interest, then it is not permissible to take a loan from either of them, whether they are Islamic banks or usurious ones.

However, if the loan from the Islamic bank is a murabaha (cost-plus financing) for a person who commissions a purchase, then there is no objection to it, provided certain conditions are met.

Undertaking a permissible murabaha is better than engaging in a usurious transaction, due to the gravity of usury.