Hi there k9kreationz! I actually had this exact same question about a month back!
My puppy had a terrible herding instinct that was making me avoid giving her walks all together in the city. With fast moving cars, bikes, and skateboarders around here, my puppy would have crazy urges to lunge at them when they would wizz by. I knew I had to do something different to get this under control. On a bright note, we DO have it under control and can now enjoy 2 hour walks, even along highways with no pulls
I used a regular harness for Precious before, but realized it wasn't doing anything to help her and in fact was only enforcing her to pull harder against her body harness. After talking to my trainers at my training facility and a few other trainers in pet stores, and of course asking on Sheltie Forums as well, I was recommended to use the Easy Walk™ Harness by Premier (it's pretty much avaliable in any pet store so it should be easy to get your hands on to, or you can consider buying it online). Everyone said no to the gentle leader (face one) because it seemed ridiculous to use it on a puppy, and that the correction could be made in an easier way than having something wrapped around their faces. The easy walk harness, though does an amazing job in making dogs stop pulling, should not be used as just a quick fix. With it's AID, it should be a LOT easier to train your dog how to heel/walk on a loose leash without the pulls (leash clips on to the front of the dogs chest instead of the back of their neck like other regular harnesses, which is what makes the difference). You can read more up on the product itself if you google it!
As for your consideration about the martingale, I would strongly suggest you also purchase one of these instead of the flat buckle, as experience of having two shelties that both had small heads but enormous amounts of fur (made it impossible to get it tight enough, they were still able to both get their collar off by accident). The martingale, again from the way it was designed, directly helps with this issue and is very useful for sheltie owners in my opinion. It's a lot safer in my mind than a flat buckle for any dog, regardless of breed. However, I do not think this is a good training tool to use if you wanted to fix pulling in dogs. It is better for obedience training, because you have better control of the neck and head. If you wanted to continue using your martingale by all means, I myself use both at the same time. But if its an immediate behaviour you're having trouble with, I'd say consider using a walking harness for some time to fix the behaviour before it becomes too out of hand or further enforced as being 'okay' before using a regular collar or martingale again. You shouldn't worry about your CGC testing now as I don't think a training aid is ever permanent. Just like the clicker, I think once your dog is able to walk correctly and is trained what proper walking behaviour is, you WILL be able to take away the training aid slowly! AKA move back to the martingale in time for your testing.
If you want to know of my experience, I ended up going to the pet store and immediately buying BOTH! Easy walk for walk training and martingale for indoors training as we do a lot of obedience together. However, after Precious has gotten a lot better with walking, occasionally (if its for a short stroll, or as you said for bathroom breaks, etc) I just use the martingale now and have had no problem because her pulling has been fixed with the easy walk's use already.
I wish you all the best and really hope you will consider buying a walking harness over a flat buckle collar. I never paid too much attention to those products because I already had alot of collars and regular harnesses at home. But it really was an amazing investment, It's like a miracle happened over night and Precious's behaviour really has changed a lot just from having a walking harness. Give us an update if you end up getting one! If anyhting, you can purcahse, keep the receipt, and try it out yourself or try it out around the store. Return it if you don't like it! Hopefully it works for you too though! Hope my story helped in some way to explain the difference!!
PS: "I noticed he's more reactive with a collar on, versus a harness"
It's probably not that he's more 'reactive' with a collar versus a harness (that more closely wraps around the body/chest for better control) on but more so that collars give dogs more room to 'turn around' and more freedom over all to do things. For an untrained or reactive dog, this could be chaotic (as I've seen in my puppy hahaha before realizing she was going to be so reactive to cars). Just think of a string holding on to the dog on one spot (Collar) compared to 5 strings (harness) holding on to the dog spread evenly apart from the middle of their body
Hopefully you get what I'm trying to point out!
