Well as the absorbtion coefficient is per cm, i am guessing the higher it is the more light is being absorbed when travelling through a cm of the substance. So the optically diluter substance is probably the one with the lower coefficient.
I agree, unfortunately I'm not comparing two spectra, but rather just answering for a variety of spectra is the solution optically dilute? Is it Optically thick? And not sure where the cut off is, can't find a value anywhere