Something to keep in mind about the stoker's bars - while your hips may fit between them while standing, your experience on the road may be slightly different.
1) Your stoker's hands on the bar will narrow that width somewhat.
2) When you put your foot down while waiting at a light, you may find the bar restricts movement to the side.
3) When leaning into a corner, if you rotate your inner leg and point your knee into the turn, you may find the bar restricts your movement.
Your mileage may vary, but the above was my experience and caused us to go up a size. As for the captain's bars, go with what's comfortable for you.