Relay-Relay circuits: flywheel diode - yay or nay?
I would say diodes are virtually always better, ignoring the slight complexity increase and minor cost (pennies for parts or fractions thereof), unless there is some reason to get every fraction of a millisecond speed (and then you shouldn't be using relays at all in the current year). 

Contact life reduction due to slower drop-out is not a serious concern because relays of normal size do not draw significant current for most power relays. i.e. you are using a 10A relay contact to switch 60mA. So if your PLC is switching the coil of a DC power relay, I say put the 1N400x in there and it will last much longer. Or use a solid-state output. 

Yes, you can use the zener + diode or resistor + diode and it will give you a bit of a compromise (more sparking but faster drop-out), but it's not all that often necessary, and it could cause more issues with field repairs because it will be unfamiliar to service people.