Being risk-averse with respect to wealth utility is reasonable, and is empirically verified to be the case with most people. Wealth utility is a special case of the more general concept of utility regarding outcomes. Risk-averseness is reasonable for wealth utility because the risk is personal. The risk that the donation with the highest expected saving of lives in fact saves fewer lives than another donation is not a personal risk. So, I agree that, assuming accurate information about the probabilities, you should donate to get the maximum expected bang for the buck.