Sandrine, you seem to be all over the place, lens-wise. The two lenses you are considering are very different in that one is a prime, and one is a zoom. One needs a prime generally because they are faster (meaning wider aperture) and have a better IQ. I would also say they are smaller than zooms, but the Sigma 24mm f/1.8 is large for a prime.
Now, you either need an f/1.8 lens, or you don't. If you're going to shoot indoors without flash, then the f/2.8 of the zoom is just not fast enough.
I own a Tamron 28-75mm f/2.8 and the Sigma 24mm f/1.8, but they serve completely different purposes. The Tamron is my most used lens by far, but when I need to photograph indoors without flash, I put it away. Why? Because f/2.8 isn't fast enough. So then I bring out the fast primes.
Here you can see a gig I shot with the Sigma. The shots without flash are all wide open at f/1.8...and ISO 1600, the max on my K10D. f/1.8 was barely wide enough and I would have preferred a 24mm f/1.4 (which only exists on Canon mount, and for a small fortune).
Regarding Eigengrau's comments about sharpness, I agree that it isn't the sharpest wide open, but it improves substantially by f/2, and my copy is certainly very good by f/2.8 and better than my Tamron zoom is at that aperture. One thing to consider is that many wedding photographs don't necessarily need to be razor sharp, so a slight softness might actually be a positive thing.
Peter Zack u owns the 28mm version of the Sigma f/1.8 and uses it extensively for his weddings. I'm sure I've heard him say that it's one of his workhorses for photos inside the church.
As has been mentioned, flare is a b!+ch with this lens, so you either avoid it, or embrace it and use it artistically:
Your decision is easy, Sandrine: Do you need the wide aperture or don't you?
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