I've run 40's to 46's and I can feel a difference when out in the wind, with the wider bars. So if TT'ing is important to you or you ride solo in the wind a lot, consider narrower.

OTOH, the leverage feel of wide bars is way better for steep climbs and some sprinting situations. If you sprint mainly seated and spinning, narrower could be fine. I like to stand and stomp, so the wider feels better to me.