Why choose a collar: the dog on the harness tries to pull, but it does not feel uncomfortable. He doesn't care about any of your leash twitching.
The harness causes the dog's head to be held along the ground, not high - the neck output is not formed correctly, Jack Russell's back often begins to fall off, because there is an incorrect distribution of load.
If the dog has free elbows (not tightly adjoining the chest), the harness will aggravate and possibly provoke them. It is best to use a herringbone collar (expanding) or regular nylon/leather.
For puppies, a narrow collar can be used during the training period and it has a stronger effect on the dog. You can recommend using conventional nylon collars with metal buckles and carbines. They are quick and easy to put on.
Well-proven soft wide collars are half-sleeves, they do not spoil the wool, are easy to remove and dress, and teach the dog not to pull the leash. Half-sleeves are best used on adult dogs who already have a head, as these collars are adjusted to the size of the particular dog and are not regulated.
After certain types of operations, "prescribe" a harness.