The contact erosion related to the speed of the contacts opening depends upon whether you are switching an AC or DC load. An AC load generally erodes the contacts less with slow opening, since it allows time for the voltage to go to zero and extinguish the arc across the contacts. You want fast opening with a DC load to extinguish the arc as soon as possible.
So my opinion is that for AC contact loads, diode suppression for the coil is better, and for DC loads, a Zener or transorb coil suppression is better.
Edit: Now if you are concerned about the erosion on the set of contacts that may drive another relay coil, then for that I would think diode suppression of the driven coil would be best.
Interestingly, old AC wall switches used to use snap-action contacts. Now they use non-snap-action type contacts.