Against each other, I think it is highly favorable for Decidueye, simply because of Weakness and the potential to Knock Out Froakie that hit the field before they can evolve.
Pros of Decidueye:
- Feather Arrow can be used without discarding Energy, making it far easier to use multiple times.
- Hollow Hunt GX is a powerful attack in both the early game to recycle search cards, facilitating setup, as well as the late game, recycling recovery like Revitalizer or disruption cards like Enhanced Hammer.
- 240 HP and being able to use Feather Arrow from the Bench means it won't be getting Knocked Out easily.
- Has the potential to set up faster thanks to Forest of Giant Plants.
- Setting up multiple Decidueye quickly may allow you to Knock Out Trubbish before it can evolve.
Cons of Decidueye:
- Relies on other attackers; Razor Leaf works in a pinch but isn't Energy efficient.
- Can't take advantage of the healing provided by Max Potion and Rough Seas.
- Both Decidueye-GX and the EX attackers give up two Prize cards when Knocked Out.
- The hefty Pokemon line (4-4-4) will take up a lot of space.
- Loses to Volcanion, which is likely to see increased play.
Pros of Greninja:
- Pokemon give up only one Prize.
- Water Duplicates consistently brings out multiple Frogardier
- Doesn't require other attackers; Shadow Stitching and Mist Slash are effective and work well together.
- Greninja has free Retreat.
- Giant Water Shuriken hits for 60 damage a pop, totally to 120 damage if you use the Ability, Retreat to a econd Greninja BREAK and use it again, before attacking.
- Healing via Rough Seas and Max Potion makes Greninja BREAK extremely durable when it is not one-shotted.
- Consistently beats Volcanion
Cons of Greninja:
- Takes longer to set up
- Giant Water Shuriken requires you to discard a Water Energy from your hand, requiring up to three Energy per turn (counting the attack cost) to maximize damage.
- Is very susceptible to N and Delinquent late game, and more fragile overall.
- Loses to Lurantis-GX, another hyped card from Sun and Moon
I feel Greninja will still have a place in the meta because when it does set up, it completely assumes control of the game, Decidueye is a threat for sure, but it too isn't without its bad matchups, some of which Greninja can deal with favorably.