I feel like unless you are Microsoft, Google, Apple, or similar, you should opt for whatever is most intuitive to your audience, and that will be whatever convention is predominant in the other products they use. An analysis of the most rational, internally consistent usage is moot. If your customers have been trained by Apple to expect "first right" in horizontal button layouts and "first top" in vertical button layouts, it's irrelevant that this is inconsistent with English reading order of left-to-right and top-down. Unless you have the power to move the entire culture, such considerations shouldn't enter the picture.