I would go for the larger size. I've read in different places that you should measure the dogs neck and then add 2-3 inches to that measurement. Of course, that depends on the breed of dog...if you've got a dog with a thinner head than a rottie you could get away with 1-2 inches over the dogs neck size.
It's usually best to go larger...I take Ben with me and try on many different sizes...
In the
www.JBPet.com catalog, you can buy a 'toggle choke chain'. Instead of two ring, it has a toggle at one end so that you can put it on much like a regular collar instead of having to slip it over the dogs head.
As for the possibility that it could catch on something..she should only be wearing it when you can supervise her, thereby avoiding it getting caught and pulling too tight for her...