I think you have the two opposing viewpoints in the recommendations from your vet and breeder.
This is a highly personal decision.
There is some evidence that waiting til 12 months or so (til growth plates close) may reduce the risk of some potential illnesses.
There is a higher chance that you will have a dog that marks a lot more and is harder to manage if you wait until after these behaviours are ingrained.
I do feel 6 months is too early, in the balance. Vets do push for this age but most males will not really start a lot of marking etc anyway til they are around 9 months old. I did my two boys at 10 months. Neither marks except as normal on walks. But I also have taken in probably a hundred or more male cavaliers that are mature adults of all ages and neutering always quickly brought rampant marking under better control.
Both marking and any other unwanted behaviours such as humping (BUT note they may be unwanted by some of us but they ARE normal!) can also be managed through good training and management.
I do find the arguments to wait til a dog is 12+, male or female, compelling now.
But that said -- a male dog that acquires unwanted behaviour and is full of hormones is a dog that may make himself unwanted in his home, result in his being left outside (not where any cavalier is happy to be), be more likely to escape and wander and get lost/hit by a car... I don't think that's a better choice and risk to take, just because of some studies! This is where owners and there own comfort level for training and management also comes in. I feel very strongly that it is far better to neuter at 6 months than risk an abandoned, unwanted, neglected or injured/dead dog. In other words,
neutering decisions have a larger context then whether a dog has a small, fractionally greater chance of a joint issue at age 9.
Anyone doing rescue or working in a pound or doing dog training will confirm the most frequently abandoned, lost or difficult dogs are intact males -- some owners have a harder time managing and controlling them, for any range of reasons. But all males are different and some really have no unpleasant behaviours or are easily trained. A good rewards/positive methods-based trainer can give advice -- I'd look for CPDT-certified trainer around now anyway as puppy classes make a big difference for many dogs and they hugely benefit from lost of socialisation and playful exposure to other dogs and people (and learning in a distracting environment).
My recommendation would be not to automatically neuter at a given age -- just see how things go. If you find around age 9-10 that you really prefer to have the neuter done, then do it then. If you are comfortable waiting til 14 months, then do so.