Get a sony! However, be warned, It has been a pain for Sony users, the heat generated is difficult to disappate, and video with IS on can limit you to 3 minutes before having to shut down for 15 minutes due to overheat.
I'd have faith in the Canon engineers to implement this properly!
I hadn't really thought about video. Not being a DSLR video aficionado, I assume that IS would help to reduce the jitteryness of videos if you are handholding the camera? This might be where in-body IS would really shine (if implemented properly), as Canon lacks IS in their wider angle lenses. The 24-105 and the 28-135 being the only two wide angle zooms with IS.
The very How would think Canon could do better? Sony holds all the patents and has over 20 years experience with in-camera IS, they know all the tricks. The problem is carrying heat away. In camera IS requires that you move the sensor, so it is hinged to allow it to move with little power, and mass must be kept very low to be able to move it quickly. Heat cannot flow quickly thru the flexible mount hinges, and a heavy heat sink would not work either.
So, not only do we have the sensor generating a lot of power in live view or movie mode, but the IS motor generates heat as well. Canon and the others already have trouble with heat generated in live view, IN body IS makes it many times worse.
Yes, you could just not do live view, video, or turn IS off. But then there is no point to having it.