Although if you think the major funders do a good job at allocating resources, it seems that the marginal additional dollar at org X should be roughly equal in effectiveness as the marginal additional dollar at your org. Given that the tax disadvantage of earn-then-donate in the US will generally range from about 20% (SS + Medicare + state taxes; no federal income tax due to itemizing) to 42%,[1] you’d need to think the funders made at least a moderate miss to outweigh the loss in tax advantages. Moreover, your contributions to the more cost-effective org could be significantly funged, which would reduce the benefit of you making a better allocation decision.
Although if you think the major funders do a good job at allocating resources, it seems that the marginal additional dollar at org X should be roughly equal in effectiveness as the marginal additional dollar at your org. Given that the tax disadvantage of earn-then-donate in the US will generally range from about 20% (SS + Medicare + state taxes; no federal income tax due to itemizing) to 42%,[1] you’d need to think the funders made at least a moderate miss to outweigh the loss in tax advantages. Moreover, your contributions to the more cost-effective org could be significantly funged, which would reduce the benefit of you making a better allocation decision.
I’m assuming that anyone in a marginal bracket over the 22% bracket is itemizing.