The mark iii has better anti-aliasing, has a higher speed bandwidth(higher resolutions, longer record times), higher resolution in crop mode, about 3.5k in width compared to 2k width for the mark ii, a better cmos sensor(higher ISO's with less noise, etc...), and the mark ii cannot record continuous full HD AFAIK. I imagine there are more things as well.
Short answer, the mark iii is the better camera for raw and in general. The real question is are its strengths worth the 3X increase in price for you.