I would stick with the ammonia if it is working. Lots cheaper, and in case of accident you just smell clean.
I was in a department store when some knucklehead in sporting goods just had to "try" the pepper spray. The whole store was voluntarily evacuated as the "little squirt" of mace was distributed through the air conditioning vents. It didn't make anyone double over (that I saw) but you could hear a wave of coughing fits move through the store!
I would think a couple direct hits would make the dog learn who is boss. You da boss!
A buddy of mine who flies hunters and fishers into the Alaskan bush tells a hilarious story of some hikers from Seattle. His boss made the couple buy "bear repellent", one can each, because they weren't going to have a gun of any kind with them. They were heading for Grizzly country to take photos. Anyway, my friend drops them off, gives a last look down as he heads back to Anchorage and see what he thinks is the guy having a heart attack. Does some kind of emergency power glide up to them to render assistance, and finds out they thought bear spray worked like mosquito repellent. The guy had turned to his girlfriend and said "OK, you spray me first and then I will spray you" He turns around and wham, is on the ground rolling in agony!
My buddy the pilot drags him into the lake and while shoving his head underwater repeatedly yells, "You spray it AT the bears, spray it AT the bears..."
Urban legend, possibly, but every time I doubt this guy he comes up with proof.... and a wilder story.
Best of luck, keep riding! Keith