Originally Posted by Wheatfield 
You'll have a million people flogging zooms at you but...
I shot weddings very successfully for years with a 50mm lens.
31mm is the new 50, so I'd have to say the 31mm f/1.8LTD would be my choice.
Well, I've shot weddings with primes, (On a Pentax 6x7, for that matter) ...with 35mm to cover the transitions and party shots and whatever else pops up. With the artificial limitation of a single lens, I think someone with only video experience might be on a steep enough learning curve as it is without having to stick to the single focal length.
It's another good reason for the second body... sometimes you want the big quality you get out of primes, sometimes you want the flexibility.... It was a bit different with film, of course, but rule one is 'get the shot.'