I don't know what 16-35 2.8 you're referring to, because Sony hasn't released one yet. There are some strong rumors that it will become available this summer, or you might be talking about an adapted one. Either way, it would not be my choice for weddings. It's difficult to take good portraits with wide angle lenses because of distortion. Until you fully understand these limitations and know how to work around them, it's best to avoid them.
The 50 1.8 is fine, a good all-around lens, works well for portraits and opens up wide in low light. But it's a prime lens, which means it doesn't zoom in or out. For weddings this can be difficult, especially when things are happening on the other end of the room and you can't run over there fast enough. Between the two choices, though, that's the better one.
If I could have only one lens for a wedding, it would be a 24-70 2.8. The aperture opens up wide for low light work, it has a very useful focal lengths in the zoom, and the one from Sony is excellent. You might even be able to get away with the 24-70 f4 lens, which doesn't open as wide in low light, but you can compensate with a higher ISO because the a7sii is the king of low light.