Some of the lobbies have 2 or more game modes; for example, the PvP lobby has multiple PvP versions using only one lobby, Blockwars has 'Capture the Flag' and 'Core', and most of the other game modes have 'Solo' and 'Team'. If Cubecraft would give every variant its own lobby like other servers do, there would only be a slight (or no at all) difference.
About 'Prison Life', do you mean 'Prison' like most of the people know it? If so, I would love to see this game mode. Even though it's completely different from anything Cubecraft has seen yet, I think this would attract a lot of new players.