Depends how much you eat when doing either. Omad usually results in less calorie intake, I would try and do it once a week and build up, keep 18/6 for the other days if you are good with it. Also eat good nutritious food. Easier said than done!
Generally, the key to success for me is “do more, eat less”…
Switch it up here and there. Your body wants to maintain homeostasis and anything you do too consistently (when you want change) it will adjust to eventually, and you’ll find yourself stalling in your goals.
I would say if 18:6 is working for you and you’re comfortable with it, there’s no need to change it up. If you start to stall, or if you find you’re not hungry at the end of your fasting window, then push it but don’t push too quickly.
Like someone else said: the best is whatever you’re able to stick to!