I prefer the old fashioned leash. If I'm walking two or more dogs, I can wrap the leashes around my wrist, knotting it in a way that even if three dogs took off and pulled, the leash will not come off. I just sit back, using my weight as leverage and those fuzzy butts aren't going anywhere. But what about retractable leashes and their little tiny handles? If the dog decides to bolt there is nothing but your finger grip to hold on. If you have two dogs on retractable leashes and they tear off after some squirrel? Well good luck. Good luck indeed.
I've worked with clients, who used gentle leaders, choakers, and different harnesses. They all have their good points. One client even had me put a weighted vest on his pitbull just to slow her down and tire her out. It worked great with the minor exception -- it was a horrible fit. She was between sizes. The only suggestion for others using harnesses is to make sure the harness is secure and completely latched, so that even a dog who decides to be troublesome can't wiggle out of a harness no matter what. I prefer harnesses that provide a squeeze and resistance to those that just hold the dog in place. I don't care for chokers, but they help when some dogs are just too strong and too in their own heads to listen to any command. Ideally, I prefer keeping the dogs away from anything that would be too arousing -- bikes, kids on skateboards, cars, other dogs, cats and squirrels. It's up to the client if he wants to introduce their dog to those elements. My job is to give the dog a nice safe environment to stretch his legs and get his pee on without any drama.
Meaning no rope burns!