I faced the same choice about 4-5 years ago. The interest rate was still somewhat higher and there was an additional bonus interest (in total over 4%) if the loan was not utilized. For me, that was the iron reserve in case additional costs occurred during construction. It still stands today and yields good interest. One just has to be careful when continuing to save/leave it standing that the building savings contract can be terminated 10 years after allocation and that bonus interest claims may expire.
as an alternative: Even if the interest rate for the building savings contract is quite high, it could still reduce the loan-to-value ratio and thus the interest over the term. If the building savings loan can then perhaps also be repaid in the short term through special repayments, it may well be worthwhile to make use of the building savings loan. This might need to be calculated accordingly.