Image quality seems unlikely to be significantly different between the two. The AF system of the T4i is a step up, but in low light, performance will only be slightly better. AF during video may be the deciding factor.
It will be interesting to see if they got that part right. Nikons continuous AF in video is awful. Sony's on the SLT cameras is somewhat better, as is that on m43 cameras.
It does appear to be the case that they designed some of their lenses with this in mind. I suspect it will only work well with lenses that are built with that in mind.