I've used the Halti or Gentle Leader head collars with a lot of success with Boxers.
When the dog pulls, the leader points his nose downwards. Doesn't hurt the dog at all.
Two potential issues though.
1. Some dogs just don't like anything on their face. Expect a minute or two of trying to get him used to it. Some will go kinda crazy trying to get it off their face. Rolling around, sulking pouting, etc. Again, its not hurting the dog. Its the same thing as how some dogs react when you put hats or christmas antlers on them.
2. its NOT a muzzle, but to uninformed passerby, some people think it looks like one. For people who are already sick of dealing with dog breed stereotypes, the look of this collar can turn them off.
I don't care for the padded harness collars at all. I've always been partial to the idea that those collars actually encourage pulling. Its like, you have a heavy puller, and you deal with it by swapping his neck collar for a pulling rig.