It's all about human-readability, if this is your main concern choose YAML:
YAML:
american:
- Boston Red Sox
- Detroit Tigers
- New York Yankees
national:
- New York Mets
- Chicago Cubs
- Atlanta Braves
JSON:
{
"american": [
"Boston Red Sox",
"Detroit Tigers",
"New York Yankees"
],
"national": [
"New York Mets",
"Chicago Cubs",
"Atlanta Braves"
]
}