Following on from the work I have done on checking what youtube does to your clips I have analysed how video is compressed real time before being saved
http://interceptor121.com/2015/02/04/demystifying-video-formats/
The full story is there but in essence:
1. Some brands have an edge on others when it comes to video compression i.e. Panasonic beats Sony why you can read it there
2. You need a bit more than 18% increase bitrate to shoot at double frame rate so that image quality is similar
3. Comparing two cameras intended for video use looking at sites like DXOMARK or dpreview can be misleading to say the least
