The best alpha cellulose fibers equal the best cotton fibers and yes both will be top quality (alpha) cellulose in the broad sense. In the trade all qualities of cotton papers can fetch a better price due to the cotton/rag name reputation. There are inkjet art papers with a top quality inkjet coating on a perfect alpha cellulose base and there are full cotton papers with lower grade inkjet coatings. There are inkjet papers with 25% cotton and the rest alpha cellulose.
The smoothest mat, satin and gloss papers in my collection are alpha cellulose based. RC papers can be smoother than that due to the plain/ECF paper base + PE barriers. It is harder to make/get ultra smooth cotton papers and there is the consumer's expectation that a cotton/rag paper should have some roughness if not at the front than on the backside.
Maybe related to the paper base content the warm fiber/baryta papers are almost all cotton, the cooler ones alpha cellulose based. Very few neutral/warm RC papers exist, almost all have OBA content and are cooler than neutral.
On closer inspection anyone will notice a difference between fiber/baryta glossy papers and equivalent surface RC papers. If not based on the ink reception side then on the back side. Tactile experience included. There are some papers in my collection that are not really falling within the two categories, either they look like RC but have no PE barriers and are based on ECFpaper qualities. Can have other kind of barriers or none at all. Low weight, very white bleached paper base + OBA content in general.
Describing the boundaries vaguely here but in practice (salesmen, ads, specifications) it can be even more confusing. Hardly anybody has touched the HM Photo Silk Baryta yet so that answer will come later.
Met vriendelijke groet, Ernsthttp://www.pigment-print.com/spectralplots/spectrumviz_1.htm
January 2014, 600+ inkjet media white spectral plots.