Wet Sponge or Brass Sponge for solder tip cleaning?
Wet sponge: Cheaper, less abrasive.

Brass wool: Removes "debris" better, smaller thermal shock (drop of temperature can be more of an issue than cracks/wear depending on your iron), you don't have to pour water whenever you start soldering.

I personally prefer the brass wool, cleans better, no water pouring, doesnt affect the temperature of the tool so much (although it still drops it a bit).