Sketching in Stillman$Birn Gamma series book.

Was so eager to try ink in my new sketchbook, I got in NY Central art supply store, that stole few minutes on the way home to sketch corner of the street in Brooklyn. I like how the ink bleeds on the paper. I am wondering what is the difference in paper quality in different sketchbooks of Stillman&Birn?  The black cover is really awesome to feel more protected on a Subway or street while sketching.