At first, I belive the preludes and fugues should be considered solely as pairs, unparted, for regarding a specific prelude means regarding its companioning fugue, and vice-versa.
The h moll (or b minor) of book I is, in my opinion, unequally complex in content and scale, showing full development of a chromatic subject (of the fugue) and of many sequences appearing in both the prlude and the fugue.
The prelude, written in trio sonata form, contains a "walking bass" and a two-part invention in the upper voices. The fugue, quite revolutionary for his time, with its subject containing all 12 tones of the chromatic scale , and contrasubject which develops throughout the different expositions just like a schoenberg theme.
studying the piece enables us to look deeper into ot and enjoy the masterpiece even more!