Wider bars will give you more room on top if you mount various gadgets on them. On my Bike Friday, I have a computer, air horn, and Halt holder, and barely enough room for my hands. Since I do a lot of utilitarian rides, I wish they were wider on top as I do like riding on the tops quite a lot.

My handlebars were 38cm Salsa Pocos, but when Bike Friday cuts them to make them foldable, they lose a cm, so are around 37 cm C-to-C. My shoulder width is around 36 cm.

Narrower bars are also a bit more twitchy; wider bars are more stable.

I would size up just a bit (within reason) due to the above. YMMV, of course.