Leaderboards is bad idea, especially for new users, i have experience with such leaderboard system, the player is always same person for each periode. Yes it will indirectly increase active players, but just old players.
Not only for new users, a gambling site should aware of their old and loyal players too. As we know, casino owner surely want his business run in long term, and without their loyal players, it will hardly happen. So, I vote to leaderboard.
For new users, bounty rolls and jackpot will be great choice.