That’s really awesome and exciting.
No right answer, of course, and the questions itself is flawed because neither are strictly necessary, but assuming they are, it’s an interesting question.
I think writing good rules is WAY easier and VASTLY less risky than writing good adventures.
Adventures are messy. You can test elements of them, but there’s no “success”, only fewer people that it pisses off. Rules can fall into the zone of mediocrity with less pain than an adventure.
The equivalent for adventures (“You suck as a GM”) is a non starter