Is 20W or 30W better for iPhone 14?

It won't make any difference. The phone regulates the charge. A higher wattage charger won't charge it any faster, so 20W is really all you need. But 30W won't hurt it in any way.

what you mean by regulates?

Controls.

Most users are surprised to learn that the "charger" is in the phone. It controls (and usually limits) the amount of current flowing to the battery.

It won't hurt to use a 30W.....or even 85W power adapter if you want.....but it won't help either.

Kind of like when you open your water tap you regulate how much water is flowing. Same way a phone regulates how much electricity is flowing into it. It’ll only allow so much (20W if you have the right model.)