I'd say the answer should largely depend on what exactly they want to do with the game, because both the Prime games and the 2D titles are better at different things. If it's going to focus on the teaser image that you get after 100%ing the game, however, I think I'd lean towards a first person Prime title. Prime's emphasis on atmosphere from a first person perspective and the ability to scan lore would both be very useful for a game that would probably be extremely atmospheric and focus heavily on background knowledge.