If you roll 5 dices and get three of the same dices, is it better to leave one and roll one or roll two dices?
My friend is saying that it is better to leave one and roll another, since that will give you a full house in 1/6 probability.
However, I thought it is better to roll both different dices since that will give 5/36 full house and 1/36 yahtzee.
Am I wrong in thinking that it is always better to roll two dices since yahtzee scores more than full house given that both full house and yahtzee are open?
Your calculations of the probabilities are correct. You have not quoted the applicable rules and it is only at the very end that you specify both the yahtzee and full house are open.
As I recall the rules you would prefer to fill in the yahtzee first. As you say, it scores more points. In addition, once you have one you can use another to fill in any blank (in the second half, I believe) you want. Given that, you would rather roll two dice as you have the same total chance of success and some of the successes are bigger.