Martingale/slip collar when we go to vet or out of yard but when home just regular buckle collar and that is just so she has ID tags on her at all times. I used to use harnesses for my non gsds puppies for control. I don't need a harness for my GSD puppy though.
skylar is 5 1/2 months old, I use a buckle collar for everyday walks, and training- and a harness for longer hikes, she doesn't walk any different with one over the other, she never pulls. i just like the look of the harness on her
Until I got my Alaskan husky I always just used a collar. I never quite understood why I would need a harness. However, the harness is helpful with the husky. It doesnt prevent or improve pulling, but since she is soo strong and pulls so much the collar was uncomfortable for her-- but didnt stop her from pulling. The harness also has another benefit- I leave it on her all the time and when she is trying to get out the door, or jump on a visitor, or even just when she is the way, I can grab the harness and conviently move her much easier than using just a collar, and much safer. However, my other dogs would have no need for a harness, as they are well behaved.
Use both so your dog is OK with either one. I do search & rescue and was initially shocked at all of the dogs/dog owners who never used a harness, and then their dog was having a very difficult time adjusting. We've used all sorts of harnesses; just about every style that goes on this way and that way. We don't use them much anymore for walks or runs, but if and when we do, they have no problem putting their heads through the holes or whatever.