according to this source
..By 1976 it had become evident that existing industrial infrastructure was inadequate to mass produce the expensive titanium hulls of this class, and that consequently production rates would not meet force level requirements. Consequently, the Akula attack submarine project using a steel hull was initiated. Construction of the Sierra class was halted in favor of the [probably] less expensive 'Akula' class.
the Sierra class submarines should be more expansive (and higher defence value) than the Akula class submarine because of it hull quality. While the Siera hull was made of Titanium, Akula was made from steel. Therefore the Siera should be more expansive but also have better defence.