I work at Zynga. If it's a cash/premium reward then you'll get the most sign ups for the biggest reward given to a few people. People consistently overcompensate for games of chance. The best is when they can take actions to increase their odds or tickets, like post on facebook, invite other people, etc.

And depending on what your reward is, I wouldn't be representing Zynga well if I didn't say you should test it. :-)