Gameplay always comes first for me. I love KH1, its the reason its my favorite video game series ever. But its a clunky ass game thats incredibly imbalanced and regardless its story or worlds, that alone makes it not as good for me. In regards to the worlds, some KH1's worlds are good but its a mixed bag and the clunky controls make pretty much all the platforming unfun which detracts from the worlds. KH2's are less interesting and far more linear in nature, but is made up for by being less claustrophobic which makes the combat and camera better. The story is obviously simpler in KH1 and thats probably a good thing. Easy to follow, easy to care about it, etc. I personally like all the bullshit that happens after KH1 but its understandable why people like the first games story. Also KH2's music is better, thats a fact.
And the biggest thing of all, that separates the good from the absolute pinnacle of ARPG's, and what makes KH2 the best KH game and the best in the genre, is FM+ Critical Mode. The only time in the series they ever looked back at what they had, created a difficulty mode from the ground up to create a perfectly tuned, perfectly challenging mode that makes you say:
"DAMN, THIS IS GAME DESIGN!"