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.
Im so sure that each of us here have different taste on this things and it matter on what type of business are you going to build on since if you have passion on gambling im pretty sure that you would surely build a gambling business right here and if you want to be a cupid surely you will also build the dating site aswell and profits on this sites are matter upon on what is your business strategy that you will implement on this.