My prefference would 18-55 and mount the flash permanently on the cam. From what I experience, being the camera man you tend to be right there on the spot and you'll need a shorter focal length.
If you're photographing in a church, it will be the other way round where you're not allowed to get in way of the ceremony and that's where a longer focal length will come in handy.
So it's pretty much depends on the situation and condition.