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Both are bad
Heres the situations
X comes in as non mega. Gets killed from rocks before you have a chance to evolve it
X leads. You use up a turn to evolve, then you use up a turn to set up hazards (if he has any), or switch . You are 2 turns behind your opponent
Y comes in as non mega. Dies to rocks
Y leads. You use a turn to evolve to get sunny. You wasted a turn, Ninetales is better