Where the pressure goes is the whole question
A collar concentrates everything — your dog's forward drive and your answering pressure — on the front of his neck, over the throat and windpipe. For a dog who walks calmly beside you and rarely pulls, that load is small and mostly harmless. For a dog who lunges at squirrels, pulls steadily, or startles at traffic, repeated pressure on the throat can cause coughing, gagging, and real discomfort, and it is a poor choice for dogs whose anatomy already makes the neck vulnerable: small toy breeds prone to delicate windpipes, flat-faced breeds whose breathing is already effortful, and any dog with a known neck, throat, or spinal problem.
A harness moves the load to the chest and shoulders, structures built to take it. That is why harnesses are the safer default for pullers, puppies still leaping about at the end of the lead, elderly dogs, and dogs recovering from neck or back issues. A front-clip harness, which attaches at the chest, also gives you gentle steering: when the dog pulls, he is turned slightly toward you instead of being rewarded with forward progress.
Where collars still win
A harness is walking equipment; a collar is identification. A collar worn throughout the day carries the ID your dog needs if he slips a door or gate, and a dog walked only in a harness may be wearing no identification at all on the occasions he gets loose at home. Collars are also quicker for a calm dog's short garden trip, and some dogs settle better in one for training classes where a harness's snug fit distracts them. Many owners use both: collar on all day for ID, harness clipped on for walks. That combination is a sensible default for most households.
Fit decides safety more than the label does
A badly fitted harness is not safe merely because it is a harness. It should sit clear of the armpits so it does not rub the soft skin there raw, allow the shoulders to move freely — restrictive straps across the shoulder change how a growing dog moves — and be snug enough that a backing-up dog cannot wriggle out of it, a classic escape in car parks. Check the fit regularly, especially on puppies and dogs gaining or losing weight.
A collar needs the same care: flat and comfortable, fastened so you can fit two fingers underneath, no looser. A collar loose enough to slide forward can slip right over the head when a dog pulls backwards — the exact moment you most need it to hold — and a collar left too tight rubs and mats the coat underneath. Check the buckle and stitching on both collar and harness as part of routine care; webbing wears invisibly before it fails.
Match the choice to your dog
Puller, puppy, flat-faced, tiny, elderly, or neck-sensitive: walk in a harness. Calm adult who strolls without pulling: a collar is reasonable, with a harness still worth considering for long or exciting walks. Nervous escape artist: a well-fitted harness plus a collar as backup. Whichever you walk him in, his identification should be unmissable, and KinsLe's Verified QR ID Pet Profiles put a unique QR code on your dog's collar that a finder can scan to reach his profile and you, even if his harness is hanging by the front door.