The harness I have used have been Bridgeport, Dean & Taylor and Bold Lead Design.
There is a whole discussion going on about a "Y" front verse a straight front in the service dog world. The question is which one is better for the dog, how the harness works, freedom of shoulder movement, etc. So please remember every dog is different, this is what I found working with Spirit (Standard Poodle, narrow build, deep girth, boney chest).
He is narrow, though his girth is right at 29" around. The Bridgeport harness is adjustable at the top of the "Y", the piece that runs between the legs and then the double strapping on the barrel of the dog. What I found was that the spread of the handle was too wide for Spirit. If you have a rounder dog (lab, pitty mix, etc) the spread works for those. The downside of this harness is that the handle does not fold down. Since there is no way to make it narrower, when I cinched everything down, there was still a twist when I used it, causing sore points in the sides of the dog. Cost around $90.
After talking to both the manufactures of Circle-E design ("Y" front) and Bold Lead Design (straight front) I decided to go with the BLD. The main reason was the rotator cuff injury that I have. When my arm is relaxed and at my side I have a twist to it. Circle-E no longer offers an offset (or "Z") handle and BLD does. I like the fit of the harness since it is tailor made. The handle is adjustable up or down of about 3", the handle also can be folded down. The grip is just the right size for my small hands and is well padded. I like how the rear cinch strap works and I do not get any torque or rotation when I use this harness. In the up position I use it for balance on my bad days (always pulling up since a dog should not have downward pressure on it). When it is laying down or if I use a rear strap, it becomes momentum pull for my good days. Having got the "Z" handle option, that eased the pain for me, when using a rigid handle. Cost around $500.
While waiting for the BLD I went to a Pet Expo and wandered into the Dean & Taylor shop. I ended up picking up this lightweight harness. I really like it. The rolled handle is comfortable in the hand. The three D-rings are of a good size and are a place to attach a small pack. It is quick on. It is comfortable for him to lay down in. The front band was a little large, but I just took off some on the strapping and then resewed it back together. This is the harness we mainly use because I am short and I don't need the extra height of a handle. This harness is used as momentum pull and upward pulling for balance. This harness is worn when I know I am going somewhere that I will be pushing a cart, doing short distances or as an "around the house" harness on bad days (so he can help me upstairs, help me balance as I do things around the house, etc). Cost around $45 at the expo with "Mobility Dog" patches on the Velcro sides.
One thing I learned, as a mobility dog, he will typically be in a position ahead of the classical "heel" position. Most rigid handle harness sit just slightly behind the shoulders, so you would want your dog walking slightly past heel position. That way the handle is remaining in the perpendicular position to the dog. If the dog is doing momentum work, they are going to be even farther ahead of you. The D & T harness handle sits about 3" behind the shoulders. I hold the handle and Spirit goes to the end of my arm reach, allowing me to rock back slightly and use that pulling sensation to walk at a faster clip. His body is always touching my left leg so he knows where I am in relation to him. So if I start to sway or twist, he readjusts to either bring me back to center by leaning away, or leans into me to straighten me up another way.
I have heard people talk about the difficulties of teaching their dog to move up past that classical heel position to work on momentum pulling. Spirit's heel position is when his shoulder is ahead of my leg, which is where the trainer set it at in the beginning.
Again, whether you are an Owner Trainer or you have a program dog, you have to take your health into consideration so compromises have to happen. One thing I had to train Spirit out of is when we stopped, he would sit. On bad days when we stop, I need him standing and leaning into me, so that had to be retaught.
Another way that they can tell "where you are" is through the leash. I have a Bridgeport leash that I sling around my body. Spirit also works in 2 different collars - a wide martingale collar and a pinch collar with the prongs softened by rubber tips. On good days, the information he gets with the martingale is enough, but if I have a bad spell during the day, I switch to the prong collar. In that he corrects to the minute changes he feels from me from both the collar input, how I'm holding the harness, to how my leg is touching him. With this type of rigging, he does slight corrections, keeping me upright and walking in a straight line. The leash is slack, but the movement in it appears to help him figure out what I need.
Again, every team is different. Some prefer a no-pull harness, others prefer a head halter. Some think that a prong collar is wrong (which if fitted or used the wrong way I can agree on), while others work their dog in a flat collar.
We need our dogs to assist us and as long as we go about it in a manner that is healthy for our dogs, we have to respect the choices of others. A true team knows that the Service Dog is vital to their way of life. That the dogs give us more freedom.