The thing is, during a wedding you're able to use flash most of the time, except maybe in the church, or when you want to capture ambience.
Assuming you're in a church service, then you will want the faster glass. You may also need faster glass if you're capturing ambience with moving subjects.
However, the "standard" ambience/mood shots you capture are of still things, such as candles and decor, and as such the IS of the longer but slower 24-105 can compensate for such things.
As Virgo also mentioned, it's price, performance and needs.