They are different type of save your a$$ tactics
Decoy; it pools the enemies in the area near by an keep them there to attack the "decoy" it stays active for a certain amount of time or predetermined decoy hit point, whichever one comes first. Mine is for 53 seconds and/or 40,200 hitpoint. This is useful when you have troops coming at you from every which way and need to find yourself a hole to escape without getting any shot.
Pros: No troop contact, all the troop and defensive buildings attack the decoy
Pretty long time and high enough hit point to let you tack control of the situation
Con: Limited area
sabotage: render small area ineffective for short amount of time, 20 seconds+/-. It's best use if you need your troops in the area where some powerful tower(s), usually my planes would fly from the wrong direction so putting an sabotage on some tower under the path. It takes some practice to sabotage multiple towers with one tactics but it's possible.
Pros: works on all defensive buildings.
Con: short and cover very small area.
I carry both one is more useful than another in certain scenario. Ymmv. Experience and practice definitely counts more than anything half of what I learn how to use them successfully were from reviewing how I was attacked and how the leaders used the tactics against me.