Precision depends on the precision of the measuring device.
Fluid ounces are a smaller measurement than cups, so a cup with a fluid ounces graduated scale would allow you to measure fractional cups of fluid more accurately than a cup that has only a cups graduated scale.
On the other hand, for a measuring container that has both graduated scales (fluid oz and cups), there is no difference in precision when you measure fluids in whole cup increments because the meniscus (fluid measurement line) would be the same on both scales.
If you want to be much more accurate in your measurements, measure by weight, for example measure using grams. Measuring by weight is better because you can avoid quantity variances that are caused by temperature-volume variations. Temperature affects volume, it does not affect mass.
50 ounces, of course!
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Ounces are more precise.
Books are typically weighed in pounds. However, if you need a more precise measurement, you may want to weigh books in ounces for more accuracy.
What determines how precise a measurement is
Unfortunately, there is not a simple answer: it depends on what you mean by "more precise". Both have three significant figures and to that extend they are equally precise. However, the maximum possible percentage errors in the two measurements are approximately 100*0.05/42.3 = 0.12% and 100*0.05/35.6 = 0.14%, respectively. On that basis, 42.3 ounces is more precise in relative terms. Also, the measurement units are smaller for 42.3 ounces so in absolute terms the measurement is more precise.
Ounces would be more appropriate to weigh a slice of pie as it is a smaller unit of measurement compared to pounds. This would provide a more precise measurement for the weight of the slice.
cups is less precise.
The measurement of 0.66 cups is equivalent to approximately 2/3 cup. To be more precise, it is about 2/3 cup plus a little more, specifically around 10.56 tablespoons. In recipes, you can round it to 2/3 cup for practical purposes.
They are both precise, but are different weights, 34 ounces is more.
Cups is more precise.
19.2cm is more precise