Not sure but the 5DIII cannot write to the SD card at the same speed the 6D can. Also to write continuous full raw HD, you either need very costly CF cards, or to lower down the resolution, bringing it into the 6D capabilities range.
Also 6D in low light is a plus. I'd say it's good when you shoot multiple cameras, to have both 5DIII and 6D. I don't think the VAF filter cost is a problem when you realise that CF cards to write full HD raw go from 350 to 750 bucks each depending on the speed.