Paper weights in the U.S. may be stated in lb., and that is determined by weighing 500 sheets (a ream) in the standard basis size of a particular paper. The standard basis size for paper varies by type, however, so measuring the weight this way is not consistent. For example, the standard size sheet for most watercolor paper is 22" x 30", but for drawing paper, the standard size sheet is often 24" x 36". Therefore, even if the sheets were the same thickness, the lb. weight would not be the same.
When paper weight is noted as grams per square meter (gsm or g/m2), measurements are based on the weight of one square meter of a single sheet of the paper, so it is always consistent, regardless of the paper type. For example, a sheet of 80 lb. watercolor paper and a sheet of 80 lb. drawing paper won’t be the same thickness since the weight isn’t based on the same size sheet. However, if the paper weight is provided in gsm, you’ll be able to tell that the watercolor paper at 170 gsm is thicker than the 130 gsm drawing paper.
So, gsm is the more accurate way to choose your paper.
Cotton papers, made from the longest cotton fibers, are generally considered the highest quality. Paper made from 100% cotton rag can handle heavy working and erasing without tearing or showing wear. High-quality 100% cotton paper can last more than 100 years. Those papers made from shorter, lesser-quality cotton fibers can become fuzzy with reworking. Cellulose papers are usually made from wood pulp and have a natural acid content that will destroy the paper over time. The more acidic a paper is, the shorter its life expectancy. Buffers are often added to make a cellulose-based paper pH neutral and extend its life.