Some museums, conservators, and framers prefer to use 100% rag, buffered boards instead of other alphacellulose boards. There certainly was a time when 100% rag board was superior to other types of pulp. But is it true today? Some say manufacturing technology has advanced to the point that 100% cotton no longer holds a technical advantage over other alphacellulose.
By definition, alphacellulose is acid free, lignin free, purified pulp and it doesn't matter whether the pulp comes from linen rags, trees or cotton plants, or some combination of purified pulps.
The U.S. Library of Congress makes no disinction in favor of 100% cotton over alphacellulose of other composition. I've looked for definitive data (lab tests, for example) to prove one composition of alphacellulose is better than the rest, and have not found it. The data I've seen indicate they are equal in terms of preservation quality -- regardless of personal preferences.
In favor of 100% cotton: It requires less processing, so the fibers are slightly longer, making the board more flexible; it feels (and is) softer. I think (but not sure) unbuffered cotton board has the simplest chemical composition of all preservation boards, which might be a framing advantage sometimes.
In favor of other alphacellulose: More processing means the fibers are shorter, more consistent, and the board is more dense (harder, stiffer). These characteristics make the boards less reactive to changes in temperature and humidity.
I like the Crescent Select series. It's excellent quality matboard at competitive prices. I also use Bainbridge ArtCare boards, Crescent 100% rag boards, and others.
While some folks claim one particular label is superior to all the rest, I haven't found reasons for that other than personal preference.