It depends on what you're seeking. As I said, from a literary point of view, the
Sins of the Fathers novelization bears little value and basically functions like an extended walkthrough of the game, thankfully enriched by many interesting insights into Gabriel's psychology, his background and his relationship with Gran and Mosely, and Malia of course. So, if you're a fan of the game and are searching for a companion for the adventure itself, the novel is certainly worthy.
The Beast Within novelization is far more accomplished as a novel per se, and Jane, chosing to overcome the game in favor of a more genuine storytelling, changed many details, effectively creating an addicting alternative version from the canon of
The Beast Within. Of course, the plot doesn't change, but many developments are presented under a different light and this value alone makes the novel worth reading. Plus, free of budged restraints, Jane was able to reintroduce in the novel the "lost chapter" that was to be played from Ludwig's perspective and that was subsequentely but due to budgetary reasons. That chapter - which later became Chapter Six opening sequence - is really fascinating, and heartfelt. All in all, I can't help but wholeheartedly recommend the second novel