The answer is early dedicated training so they don't pull from the off. It's never too early to start lead training your puppy, even if they can't go on the street yet. Spend time now on heel work and basic commands and it will pay dividends.
As soon as he's vaccinated, get to some good puppy/dog training classes. If you put the effort in now you won't have to think about a harness or collar, you'll be able to use whichever you prefer. Personally, on a cocker, I prefer a collar as a harness can knot up the coat under the arm pits, but for some, and one of mine is one, a harness is the only way to stop them choking themselves.
hth