i think a gambling site will be much better. You are more likely to have repeat customers (people who are going to throw in their money consistently).
As compared to a dating site a lot will have to go into advertisement and the location will also be a factor.
Both Gambling and dating deals with different type and mood of people and both have their own customers. We cannot compare the two. For the people, both of them are important and interesting, and choosing the one over the other is not possible.