This is not a scientific experiment. I just went out with both lenses and shot from the same spot with the D700 at iso 200.
I shot both lenses at F4 to be fair and used jpegs straight out of the camera.
16-35
		
		
	
	
		
	
12-24 from same spot
		
	
16-35 100% crop right bottom corner (almost)
		
	
14-24 crop of about the same area
		
	
From what I see, there is not much difference in quality. But the 14-24 is wider.
I'll let you make your own mind if you have to get one or the other.
Thanks for the view.
			
			I shot both lenses at F4 to be fair and used jpegs straight out of the camera.
16-35
12-24 from same spot
16-35 100% crop right bottom corner (almost)
14-24 crop of about the same area
From what I see, there is not much difference in quality. But the 14-24 is wider.
I'll let you make your own mind if you have to get one or the other.
Thanks for the view.