This is because once your head gets below the sand you automatically suffocate and the more you move the more you sink meaning unless rescued by a machine you won't survive. Also when suffocating your brain sends a signal to try and breathe so you squirm to get free wich makes you sink even further. You will suffocate before anyone gets to you more than likely. Otherwise the sand fills your nostrils and lungs and if eye's are open they star to be very painful because the sand is very packed and the pressure will damage your eyes very bad.
All the mentioned option will yield the same DNA for the suspect but blood, besides the DNA also contain further information that can be matched to the victim like blood group, genotype, some antibodies present and even traces of substances and disease.
