Shooting a wedding will be tough with a single prime lens. If you are shooting the ceremony itself you will need both a long, fast telephoto for shots of the couple at the alter, and a fast wide lens to get them coming down the isle surrounded by the guests and beauty of the church.
I've shot weddings with all primes back in my film days. The last one I used a 180/2.8 (alter, tight shots), 24/2.8 (wide shots, reception action shots), 35/2 (informal or group portraits, anything where the 24 was too wide), 105/2.5 (most posed portraits).
If I had to go with a single prime on a crop body for a wedding I would take the 35/1.8. Pick the one that focuses faster/better in low light. I've used lots of Canon L glass and it has all been superb. I've never used the Sigma 35 you mentioned, but it's getting good reviews.
You'll suffer trying to get candid shots entirely in the frame, unless you back up and everything is at a blah distance from the camera, or lines up just right. I like wide lenses for candids, because you can get close and personal with the subject and still have the background included to give you the context of the event.
You will also suffer making tight shots, especially since you are shooting high iso in the low light, and will need to shoot as tight as possible to avoid cropping. Use your feet and get in close. Forget about trying to get shots from your seat.
Knowing how I shoot, I might be tempted to go with a fast 20mm on a crop body, but if you need to shoot the ceremony too, 35mm is probably the best compromise.
Have fun at the wedding.