I am leaning towards the later neuter option nowadays. Just seems like a good idea to let the dog develop as "naturally" as possible before going in and snipping out organs.
Ever compare the size of a steer to a non-altered bull? Steer get HUGE, bulls get big but nowhere near the size of a steer. GSDs are already at risk for bone/joint issue and adding any size to the equation can't be beneficial. I don't know if cow vs. dog size development correlates in any way, but it's something to think about at least.
As to the vet vs. breeder thing. That is your call. Both parties have unique knowledge. One is a specialist in the the breed(breeder) and one is a generalist with much better knowledge of physiology(vet) and many years of study and practical application of that knowledge. Ask both parties why they chose XX age to get the snip-a-roo done, compare/contrast.
Never cook bacon while naked.