The MCP23017 gives you 16 extra real IO pins. Each one has facilities such as pull-up resistors, change notification interrupts, etc - the kind of things that you expect from real IO pins. You can read and write to all of the pins.
A multiplexer though just feeds signals through to the existing pins. They're pretty dumb. Only the signal that is currently fed through can have anything done to it. So only one signal can have a value written on it or read from, and pullup resistors and interrupts are non-existent.
However the CD4067 can be used with signals other than digital - as such they are good at being a multiplexer to the analog inputs, something an IO expander can never do. You could, though, use an I2C ADC chip for that.
For high speed IO control you really want to be using the MCP23S17 instead of the MCP23017. It uses SPI instead of I2C and thus is much faster to control.