neither. the 1604 has 16 SEPERATE channel whilst the 1642 has (i believe) 8 SEPERATE channels (1-8) and 4 SHARED channels (9/10, 11/12, 13/14, and 15/16).
it just depends on what you need. the 1604 will give you the option of setting the LEVEL, EQ and PAN of 16 SEPERATE inputs. with the 1642...