Something interesting I found. This part I found particularly interesting:

A Study on Prong Collars was done in Germany:
100 dogs were in the study. 50 used choke and 50 used prong.
The dogs were studied for their entire lives. As dogs died, autopsies were performed.
Of the 50 which had chokes, 48 had injuries to the neck, trachea, or back. 2 of those were determined to be genetic. The other 46 were caused by trauma.
Of the 50 which had prongs, 2 had injuries in the neck area, 1 was determined to be genetic. 1 was caused by trauma.
I think I might change to the prong collar. The slip doesn't seem to be working for Kai's walking (it works fine for his obedience work though). I'll definately need something that tightens because he's a stinker when it comes to walking. He likes to pull backwards and has slipped out of flat collars before...I'll have to ask the trainer what she thinks.