You can remember it this way...the sifter is looking for something to eat. So he is sifting through the sand looking for food. The Shifter is just moving around and inadvertantly moves the sand as he goes.
The sifter is eating your infauna...thats bad. The shifter is eating ditritus, algae, extra food...depends on what kind of shifter you have. I would also suggest conch. Great little guys to watch!