My doctor has said by volume and others say by weight. I noticed that when I measured 1/2 a cup of meat in a measuring cup (which equals 4oz) is NOT the same when I measured it on my food scale. According to my food scale I was an oz short of 4 oz of meat. Opinions? I just want to keep heading in the right direction and don’t want to overeat but I also don’t want to under eat.
If you are looking for a measurement so you don't eat more than your stomach holds, then volume is what you are looking for.
If you are trying to determine calories, macros, and such, weighing is more accurate because how certain things fit into measuring cups can vary quite a bit, especially things like plants that might be roughly chopped vs finely chopped, or if they are cooked or not, etc.
I'll usually do both, I weigh foods when putting a meal together then I will use something like 1/2 or 3/4 cup to determine the volume to help me not eat more than I should have at one time. I try to not eat until I physically feel it in my stomach.
If you are dividing up something that has a food label, the grams should be listed on the label and you can use that for weight. Sometimes they list measurements such as teaspoons, tablespoons, cups, etc, and you can go by that too, but weighing would still be more accurate in most cases to determine calories/macros.
Obviously, for a recipe, you'd just go with whatever measurements are listed.