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.