Note that you also have to consider how certain it is that your preferred candidate really is better. And since the division of opinion is usually 50⁄50 or so, unless you are extremely overconfident, it is quite uncertain that your candidate is better. So you need do multiply the benefit not only by the probability that you will cause your candidate to win, but also by the probability that your candidate is better, and then you have to subtract the disutility in the case that your candidate turns out to be worse.
Note that you also have to consider how certain it is that your preferred candidate really is better. And since the division of opinion is usually 50⁄50 or so, unless you are extremely overconfident, it is quite uncertain that your candidate is better. So you need do multiply the benefit not only by the probability that you will cause your candidate to win, but also by the probability that your candidate is better, and then you have to subtract the disutility in the case that your candidate turns out to be worse.
In other words, don’t vote.