For HMDA you should use both if both contributed to the denial.
When using an "arbitrary" score when the applicant qualifies in every other way is sometimes difficult to defend.
From the Commentary to 1002.9(b):
Disclosing the key factors that adversely affected the consumer's credit score does not satisfy the ECOA requirement to disclose specific reasons for denying or taking other adverse action on an application or extension of credit.
You have to be a bit more specific in your reasons than the generic terms used by the CBRs for the factors.
BTW, if one of the reasons is o/s balances in relation to credit limits (or however that term is worded) and you do not have an allowable (acceptable) percentage in the loan policy, that reason would not be valid because you have no set ratio defined.
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The opinions expressed are mine and they are not to be taken as legal advice.