Let me say I am just about to start training my first gun dog too. I be new to field training with birds but not dog training.
I always looked at clicker as a way to teach basic obedience and new tricks etc. Clicker/Marker word all same. Marker word is not a command.
I will be using a clicker for starting out teaching new behaviors until pup knows the command for intended behavior (Sit, Who, Come, etc.)
I see whistle as totally different concept than a clicker. ts not a marker sound.
Once the dog reliably knows a command (Sit, Whoa, Come) I plan is to transition and add the whistle command.
Its just another sound that means same as word command it already knows.
Once the dog understand whistle is same as the word command it can be used at distance.