You can make profits with both ideas but an online casino won't be making money right after you made the website. It also needs a bankroll to start with. You'll be also needing high grade security. Even if you manage to have all those, there can always be a lucky customer and if you don't pay him/her; he/she will ruin your reputation on these forums. (Like the one happening with sportsbet.io nowadays) And your casino will be making no money.
A dating website is on the other hand looks easier to succeed. All you have to do is promoting your website. Also have nice design, put some fake profiles at the start and you are good to go. You can't fuck it up even if you wanted to.
For years I have been contemplating to build a dating site or to be an affiliate because of the high pay associated with it. But I never got the just to put one. So for me, dating site is much more easier to promote and succeed because a lot of single men are looking for a partner any young and attractive lady. Unlike in gambling, its hard to find big players and the market is almost saturated now.