Cognus was cool I really like her story in Bane Dystany of Evil. I see no real reason to choose tenbrus as we don't know much of anything about him at all.

@Darth Malice24 Agreed. Plus we know why exactly she joined the Sith.

The only time we got to see Tenebrous in action is where he's killed by his apprentice. Not saying he's an uninteresting character since the Darth Plagueis-novel does reveal more about him (his intentions, his backstory) than only his death. But I personally didn't find him all that impressive in the one scene he appears in.

Cognus, on the other hand, was a much more active character in the final part of the Darth Bane-trilogy. Although I don't remember that there was all that much information on her background, she was an intriguing character. Encountering force users of several different affiliations, working initially for Serra, managing to kidnap Darth Bane himself, only to be apprenticed by him later on, then finally becoming Bane's own apprentice's apprentice. I personally found her, based on those parts, more intriguing than Tenebrous. Again, not saying the latter is a bad character, only that I prefer Cognus

Wasn't Tenebrous known as one of the clunkiest and weakest sith lords ever? Thought I read that somewhere.