Go for the 16/18-55, hand down. In all honesty, the difference between 50 and 70 is barely noticeable, but the difference between 16 and 24 is huge.
Yes, I have to concur with the General here...go for the f/2.8 zoom with 17 or 18mm on the bottom end and the 50-55mm top end; the 24-70mm lenses on 1.6x are very restrictive in small locations. If you need to do a group shot, with a 17 or 18mm you can include a wide view from a "normal" distance; with a 24mm that is roughly a 38.4mm equivalent.
Here's a rule of thumb for full-format photography, but it applies to a 24mm setting on 1.6 x just as well; With a 35mm focal length lens on Full-Frame, the angle of view is roughly one foot wide for every one foot of distance to the subject. On 1.6x, a 24-70mm lens isn't really a wide-angle to short telephoto lens; it is a semi-wide to short telephoto lens.
The reason the "kit" lens is now an 18-55mm lens is that is the angle of view range most people feel they "need" for a one-lens setup; the constant f/2.8 aperture 17-50 Tamron and Sigma 18-50, and the Canon 17-55 f/2.8 and the Nikon 17-55 f/2.8 DX---those four lenses are evidence of what is "needed" as a wedding-event lens on 1.6x...f/2.8 aperture, and the right range of lengths. For longer working distances, the 24-70 is fine, but if you only have ONE lens length, I think I'd go for the 17-50 f/2.8 on a 1.6x cam.
If the event is outdoors,exclusively, and I wanted to shoot more posed formals, I would obviously ditch the 17-55 in favor of a 24-70. One small tip: with a 17-50mm, it helps to squat down bit when shooting, to keep people from looking distorted; you need to keep the back of the camera parallel with the spines of the people when shooting wide and close; just a few inches lower a camera position will improve 17-50 people photos,especially those done at the shorter ranges at shorter focal lengths.