Canon 5D mk III vs Sony A7s
Forward thinking the A7s is capable of 4k (with external recorder, e.g. atomos shogun) which is trendy and can shoot at a higher fps (120 fps) and in S-log which for me is three major selling points over the 5D. Not to mention the picture is a lot sharper. The 5D in my opinion is not something I would want to use given the option of the A7s, the image is soft and muddled in comparison. I have used both.

That being said, if you are not concerned with taking pictures, and strictly want to get into video may I suggest buying a C100 MK I. On ebay they are in roughly the same price range, and the camera, although not out performing the A7 in terms of resolution and frame rates, has a beautiful Canon look out of the box, professional video controls + form factor, and many people prefer C-log over S-log.

That being said, if I were to get a B-cam to my C100 MK II, I would get the A7S. Sure it might take some work to match the Sony green-cast to the generally more neutral Canon image, but the pros far outweigh the cons.