The main difference between the two is that the Psr-600 will receive digital frequency transmissions, and the Psr-400 will not as it is an analog scanner,,both do trunking and are great scanners ,and there is some feature differences..I just bought the Psr-400 and its a great scanner, I don't have digital in my area, or trunking ...But id still like to have a 600 some day,,i love feature loaded scanners, and Gre makes excellent scanners.Anyone want to explain the difference in these two? I am much more interested n paying $200.00 then the $400.00 plus.