Dhelmise and Decidueye are so similar pokemons. Both are Grass/Ghost, are pretty slow, are being used as spin blockers and have Hazard Removal moves. My question is: What is the difference between the two as far as it's uses goes?
They also both have base 80 physical trapping moves and their names begin with D!
There are a few differences between them. Decidueye is pretty slow, but Dhelmise is unsalvagably slow. There's almost no reason to run any Speed on it, whereas Decidueye can invest to outpace walls and slow offensive Pokemon. The difference between Rapid Spin and Defog is also pretty major--Rapid Spin is more useful on hazard-stacking teams where Defog is counterproductive. Dhelmise is also hard to spinblock because it threatens most Ghost switch-ins other than Mega Sableye. Defog, however, is more reliable if you don't necessarily care about your hazards and want to avoid all Ghosts/contact damage. Dhelmise is a bit more physically bulk while Decidueye is a bit more specially bulky.
Dhelmise's true strengths are its massive Attack and pseudo-STAB on Steel moves. Anchor Shot, unlike Soul Shackle, has no immunities and hits Fairies super effectively, which is important in such a Tapu-heavy meta. It also has Switcheroo, which I've seen used on Choice Band sets to cripple walls like Skarmory. Overall, I see Dhelmise being used on bulky offense or Spikes-based teams where its immediate power, access to Rapid Spin and a more reliable trapping move can make up for its lack of Speed and recovery.
Decidueye, meanwhile, is a lot more diverse. Its Atk and SpA attacks are both pretty decent, bolstered by the potential to run either Swords Dance or Nasty Plot. The currently dominant sets, however, take advantage of the utility Decidueye brings to the table, often running a specially defensive spread. The ability to run a defensive set despite its merely-okay defenses is thanks to its access to Roost. U-turn is also a great asset that can grab momentum, sometimes in cases where you've trapped a foe and U-turn into a counter with no fear of a double switch. I've even seen people toy with options like Curse and Decidueye's signature Z-crystal to unleash a powerful physical Ghost move. Overall, Decidueye somewhat suffers from a lack of specialization, making it potentially mediocre no matter what it runs, but the diversity it exhibits compensates for this fact and allows it to shine a variety of roles. I've seen it used on Offense, Stall and everything in between thanks to its unique set of traits.
For what it's worth, I doubt either will stay OU for more than a few months, but each of them is solidly usable in the current metagame.
