On a trail I would opt for pepper spray if a dog came at me. Dog behavior varies considerably depending on how it was trained, the disposition of the breed (guard dog versus sheep herding breeds) and how many dogs. Dogs will be more aggressive in a pack and in Marin county in California there was a big problem with people letting their dogs run free during the day. When the dogs formed into packs they would chase down and kill deer, especially young fawns.
I have breed and trained many sheepdogs over the years and my experience has been that most dog owners make no effort to train or learn how to control their dogs on or off a leash. I was in a park in Anchorage a few years ago and two women had their dog off lead and it was chasing down a moose with its calf. The women thought it was very funny and I regretted not having a cannister of bear spray with me.
I have not been able to ride for the past month as my neighbor's dog got out into the road as the neighbor does not have fenced yard and only an electronic perimeter wire that requires that the dog have a special collar on its neck with working batteries and the neighbor could not be bothered to use the collar. To avoid the dog I crashed my bike on the pavement and tore up both knees and an elbow and a wrist. This was not an accident but the result of the negligence of my neighbor as a dog owner. A dog that causes a bike rider on a trail to crash could result in broken wrists or collar bone and the dog owner is not going to pay for the rider's ambulance and hospital bills. In areas where ranching is occurring any stray dog can be legally shot by the rancher. I feel sorry for the dog when this happens but not the dog's negligent owner.