How experienced a filmmaker are you, and what kind of films are you planning to make? For journalistic/documentary/news the 24-70 would be good, though I think autofocus will be slower than on a Canon body. I'm not convinced that it's a good idea to use a lens developed specifically for Canon on a body made by another manufacturer. I know the A7S is small and very good in low light, but I really like Canon colours and usability. On your budget, and for pro use, I'd go for a Canon C100 unless still photography is important to you.