Start with this thought:
- If I have on-balance stronger reasons to do A than to do B, and I am choosing between A and B, then it is better that I do A than that I do B.
- If in decision X, I choose A over C, and in decision Y, I choose B over D, and I had on-balance stronger reasons to do A than I did to do B, then decision X was better.
There is a whiff of paradoxicality here. But it's just a whiff. If you chose your convenience over your friend's life you'd be a terrible person. So in a case like that described in (2), choosing B (e.g., your friend's life over your life) is a better thing than choosing A (e.g., your friend's life over your convenience), choosing C (e.g., your convenience) is worse than choosing D.
In other words, when you choose A over B, the on-balance strength of reasons for A doesn't correlate--even typically--with the value of your deciding for A. Rather, the on-balance strength of reasons for A correlates (at least roughly and typically) with the value of your deciding for A minus the value of your deciding for B. This is quite clear.
This helps to resolve the paradox of why it is that doing the supererogatory is better than doing the obligatory, even though in a case where an option is obligatory the reasons are stronger than the reasons for supererogation. For omitting the supererogatory is much less bad than omitting the obligatory.
We may even be able to use some of the above to make some progress on the Kantian paradox that a good action by a person with a neutral character is better than a good action by a person with a good character, once we observe that it is worse for a good person to do something bad than for a neutral person to do the same thing, since the good person does two bad things: she does the bad thing in itself and she fights her good personality. Thus, even though the good person has more on-balance reason to do the good thing, because the strength of reasons doesn't correlate with the value of the action but with the value of the action minus the value of the alternative, this does not guarantee that her action has greater value than the good action of the neutral person.