Didn't care for Sion at all, but I loved Nihilus, primarily due to what he was in relation to Kreia's philosophies. In my eyes the game's story is about how the Force relates to our human emotions, and Nihilus plays an important role. He's not so much a person as much as a notion.
Agreed. KOTOR II set forth ideas that made people think. While many of these ideas have been discredited in the grand scheme of things, they certainly make one think.