The video on A57 is better than any Canon Rebel or Nikon DX series, thanks to aperture control in live view, auto-focus in live view, and focus peaking (though, for Canons, you can get focus peaking via Magic Lantern firmware, plus a lot more cool stuff).
Although A57 has a bigger sensor, it is still not as good as Panasonic DMC-GH3 in video quality, so if you're buying mainly for video, I would got with GH3. A57 is very good as a combo for both stills and video, but not great at either. Also, consider the lens that will work nicely for video, such as the Sony 16-50/2.8. Kit lens driven by a screw and small apertures are not ideal for video.