I get a feeling our money will be going to Sony this time around!I think the A7s was made for wedding videography. In addition to being a low-light monster, it's very small and light. If you can pair it with a light lens, it will be very easy to move around with.
I use mine mostly for school plays and musical performances, so I mostly use the Metabones EF adapter and my Canon 24-105mm and 70-200mm lenses, always on a tripod. The adapter means I have to focus manually, but I was doing that with my 5D Mark III anyway.