Actually, you do not have to make that decision. Note that your distribution, from the IRA, was reported on a form 1099-R, not a form 1099-DIV.
Although the IRA was earning dividend income, while the money was in the IRA, it is not treated as dividend income when it is withdrawn (distribution). It is taxed as plain retirement income and the type of investments, it came from within the IRA are ignored. None of the 1099-R amount is considered as coming form US government obligations.
