Well, actually it's more like that the way they struck me in their playthrough of Dark heresy is that(and I could be wrong), Geoff's knowledge of the lore comes from Codexes, Index Astartes articles, basically the Games Workshop published background material. Which, don't get me wrong, is a perfect place to learn 40K background. Especially since that is where the background comes from.
But TB seemed to have a more in depth knowledge of the lore that I've associate with someone who has read the novels and short stories as well.
I'd expect both to be able to tell me things like, what happened when Hive Fleet Behemoth attacked Macragge, or a rough accounting of the 12th Black Crusade.
But I think TB would also be able to tell me about Ahriman's mind palace, or Cyrene of Monarchia, because those are things that exist in the Black Library novels and it seems like he has read those.
Like I said, I could be wrong about that; those are just my impressions.
But since roleplaying to me is a form of proxy living in the narrative, I would prefer TB because it seems like he could create a more in depth and true feeling of 40K than Geoff can.