shute – the a7r has continuous focus peaking available during video, and of course an evf, which is very helpful.
canon's actually have better video implementation (peaking, zebras, higher throughput) once you get magic lantern (except that canons don't have evfs of course).
the new nikon sounds very interesting, i'm excited to see what it actually is. the primary difficulty with nikon (and canon to a lessor extent) is that many of the R lenses will interfere with the mirror movements unless you modify portions of the lens, camera mirror, or both. not sure if this is an issue for any of the lenses you're interested in though (you should double check).