This question already has an answer here:
For example, say I already got some Dratinis, and one Dragonair, and now, if I catch a Dratini versus if I catch a Dragonair, does it matter if I am going to transfer it to the Professor to get an extra candy to evolve my Dragonair to Dragonite?
That is, if I catch a Dratini and transfer, I get one Dratini candy, versus if I catch a Dragonair and transfer, I get one Dratini candy also? So there is no extra benefits of catching a Dragonair vs a Dratini?
P.S. if so, a strategy can be, find out how many candies you need to evolve to a Dragonite first, and once you know it is 100, then when you have 25 candies, don't evolve to a Dragonair first, because if you catch one along the way, then the 25 candies you spent can be considered a waste.