"I applaud everyone's altruism, but I'm donating the same amount to charity X instead because I think it does even more good, and I invite others to join me" is, I think, a way to talk about efficient charity without being a party pooper.
(I noticed, while writing this, the vast gulf in connotation between saying "X does more good" and "X does even more good", so tact is important.)
From the article:
Link. (Chicago Sun-Times)
I wonder if this atrocity is going to go unpunished?