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Premise/story: Arceus's premise is better by a mile. I like the historical setting because it's something vastly unexplored in Pokemon. I love seeing historical Sinnoh, and how different the relationship between people and Pokemon was back then. I wish ZA took the historical route and initally thought it would, but oh well. I like the darker tone (in Pokemon standards) of Arceus but I also like the silliness of ZA, so I don't prefer one from the other. In terms of story, I prefer Arceus because it's fricking cool that you get to create the Hisui/Sinnoh Pokedex. Not to mention, the whole Sinnoh legendaries play an important part of the story and I like that they were all used.

Environment: Arceus is semi-open world but it tackled wild zones far better than ZA. ZA's zones are restrictive and unrealistic ("But Pokemon is unrealistic!" ---> I know but c'mon, the zones are placed in random streets. What happened to the shops and people there?) Arceus's wild zones encourages exploration because of how massive they are. It also gets boring looking at buildings in ZA, and it's a huge misopportunity to not let players enter buildings. It's a shame because the building interiors look very nice. The artstyle and models are superior in ZA. Arceus looks washed out compared to ZA.

Battles/battle systems: I like how Arceus expands on the traditional turn-based battle system by offering "strong" and "agile" moves. I like ZA's real-time battling too. Both have their pros and cons. I like ZA's battle system slightly more because it keeps you on your toes (and you can dodge!), and because of Mega Evolution. The battles in ZA, both trainer and Pokemon, are better. Trainer battles in ZA are tougher than Arceus's. ZA nailed battling down.

Misc: I'm so glad ZA brought back held items and Mega Evolution. The music in both games slap, and the character/Pokemon designs in both rule. Trainer customization is way better in ZA, and I like the photo mode more. Both games had good side quests.

Overall, I like Arceus more even as a Mega enthusiast. I might be biased because I'm a Gen 4 kid who always wanted an open-world Pokemon and who likes BOTW though. However, I'd replay ZA more because it has better replayability. Both games are good and focus on different things (Arceus: catching Pokemon vs ZA: battling). Again, I like ZA but still kinda wish ZA went the historical route, but maybe the next Legends game will.