Why...?

Oh, right...

Basically, it's because I think both organizations Do Great Good with marginal dollars at this time, but the world is too uncertain to tell whether marginal dollars do more good at CFAR or SI. (X-risk reducers confused by this statement probably have a lower estimate of CFAR's impact on x-risk reduction than I do.) For normal humans who make giving decisions mostly by emotion, giving to the one they're most excited about should cause them to give the maximum amount they're going to give. For weird humans who make giving decisions mostly by multiplication, well, they've already translated "whichever organization you're most excited to support" into "whichever organization maximizes my expected utility [at least, with reference to the utility function which represents my philanthropic goals]."