The prong, what you call "inward-facing spikes," is kinder than the choke. Less damage to the trachea.
Many dog trainers use these collars. It does not signify a vicious dog. In fact I often feel more comfortable in the vicinity of a dog with the proper collar to control it than in the area of a dog with one of those feckless flexileads. Now those things scare me.
"a dog with one of those feckless flexileads. Now those things scare me."
http://www.dogsecrets.com/articles/a_4.html
Finally I found a trainer (besides my own local) who speaks up about these dumb things.
To me, since they started out in the '80s, it is the sign of a cuckolded owner who is mastered by the dog.
The other week, in fact, at NIGHT, I was driving home in the neighborhood and suddenly I vaguely see a form in front of me, then goes off to the right - an Irish Setter let to go out in the street thanks to 1 of these dip-!$#!@#% "leads" by the idiot on the sidewalk. And of course, sidewalks are generally only 4 ft from roads.
If I had been going faster I may have hit him. They were all in dark, to boot, so I barely saw him - mostly realized what it was after the fact!