Keep the 35!
I sold the 24-70L to afford the 35 Art, and I am happy that I made this swap. Sure the zoom could be really practical in a lot of situations, but the 35 is such great FL in my opinion.
I was not so happy with the wide open performance of the 24-70L, it just was not so sharp at 2.8.
The 35 is my personal favorite lens for a wedding. I use two body's and the 35 is almost all the time on one of them. Combined with a 85 or 70-200 is just great for weddings.
I can see my self using a 24-70, but I would never re-buy the v1 L ever again.
I use my Sigma 35 Art almost always at f/2.0.