I'm not sure, but my gut feeling is that the nylon dominant-dog collars have lower abrasion than the metal ones, thereby making their motion (on/off) smoother and more reliable. Or perhaps they're just lighter and easier to maintain? Then again, it could be just a "public perception issue" - ie. "dominant dog collar" is more socially acceptable than "choke collar."
In terms of using two collars, that depends entirely on your dog, situation and equipment. If you're dealing with a dog that may be handler aggressive, then your setup of prong + dominant sounds right, as prongs can and do break (especially the cheap ones), and you don't want to be in a situation of not being able to control a dog that's going after you. One caveat is that you have to be sure that a correction with the prong will not also apply a correction with the backup collar.
I also use a backup collar when I have a prong on, but the backup is either a martingale or flat collar, and it is usually not connected to the lead. In my case, the backup is there purely in case the prong breaks - it is not for immediate control of the dog.