Its pretty subjective and based on your intended use.
Typically a 9' rod is regarded as a better casting rod, whereas a 10' rod has better reach so you can mend more easily and it is better suited to nymphing.
The obvious down side being that 10'ers are pegged as not casting as well as 9'ers.
That being said, you will find a multitude of accomplished fishermen who will either use a 9' stating that a 10' offers no advantage for them or a multitude of accomplished fishermen who say 10' is the only way with no disadvantages being noticed.
Ultimately its up to you, your skill set and its intended application.
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