Do you first decide on story or gameplay?

I would say neither, I tend to view my projects as two interconnected skyscrapers.

As I build my story & the gameworld's lore, I am also building upon the gameplay concepts & components, & each shapes the other. As the project develops, both are changed by those interactions,

I usually start with gameplay and mechanics. Then I build the world and characters around that gameplay, to some extent. Finally, I craft a plot that makes sense for the characters and world.

I agree that there are multiple good ways to start and build, though!

In an jrpg I think it's better to start with the story, it feels more important to me.

First, start with knowing your purpose about the game. In fact, don't just "know" it. Say it out loud. Summarize your game in one sentence. Since one sentence can't include everything, you need to prioritize. What do you find yourself focusing on?
If you feel you can ignore the specifics of game-play but you can definitely not ignore the story, then story it is. Or maybe vice versa.

From then on, I think they should go hand to hand. Depends on your experience too, of course. Let me gamify.

Let's say your game's main class is Story.
Initially, both your story and game-play are level 1.

You'll want to start with story, get to, say, level 3 or so. Before you advance further, you want to get at least 1 level in game-play. Then let's say you move to level 5 with story. Well, make sure game-play is at least level 3.
Because if there is a big difference, it will be harder to fix any issues.

This is where experience comes in. If you know your capacity in terms of writing, editing or creating-scripting game-play, then maybe you know you'll be fine with a big difference... or maybe you know you should always keep them at the same level.

Does that make sense? :)