This is a hard one to pick because both are very good at what they do, setting up entry hazards and poisoning the foe. Neither will be a Pokemon that does a lot of direct damage through attacks, but Garbodor can deal a little more damage with its useable attack and access to a few decent moves. However these are not meant for attacking, but to cripple the opponent's team with spikes and/or toxic spikes. I think more people chose Garbodor because overall it appears slightly better and has access to clear smog, but this may not be the case. Whirlipede has access to one of the greatest items in all of competitive play, Eviolite. When holding eviolite Whirlipede's already good defenses skyrocket especially its physical defense. With this it can set up entry hazards on a Pokemon that uses physical attacks at will. It can also use toxic of foes that arent affected by toxic spikes due to not being grounded. Switch Whirlipede in on Sawk and start setting up. While most may pick Garbodor, I would strongly consider picking Whirlipede because it can do a great job. I also think Whirlipede has the better designs and also the better shiny.