I think maybe an easier way to think about this is to avoid comparing selfish and altruistic things you do (because that comparison is hard) but at least try to be effective in each category separately. Then it's fair to ask why one would buy an iPhone over an Android or why one would vote as opposed to donate a malaria net (assuming time is roughly equivalent to money).

It's not that comparing across the two categories is invalid; it's just that the honest answer may be "I don't care enough about other people to go without my iPhone" and that's not an honest answer anyone wants to give. More generally, this comparison runs up against utility functions much more than the other.