I went with 24.
My thoughts weren't much about the tiny speed improvement, but simply that my 2009 Mac Pro is doing fine with 24, so I don't believe I have much real need to get 32 in the 2012 machine right now "today".
In addition, if I do find that I need more RAM, the risk of its price going up is low, so it will more likely be cheaper if I wait 6-24 months.
-hh