Always measure in metric. It’s standardized. A cup of flour or sugar will vary depending on how densely you pack it, ambient humidity, etc…. But 100g will always be 100g.
Grams for weight and milliliters for liquids.
It does not matter whether you use metric or imperial units. What matters is whether you’re measuring by weight (i.e. g or oz) or measuring by volume (i.e. ml or fl oz). It’s much more accurate to measure by weight.