I have a couple nylon breast collars I used before I got some leather ones. One was by weaver, but they stopped carrying that set. I have one by triple e that I love. It's soft nylon with nice felt. I've never had an issue with it stretching, twisting, or any problems in general. The triple e is pretty cheap and comes in nice colors. As for bridles I prefer leather since I think it's more comfortable for the horse and they wear better.
Link to the breast collar: Triple E Nylon Trail Breast Collar - Horse.com
And a pic of Jinxx wearing the triple e breast collar. Please excuse the mis match. I was putting everything I could on him to get him use to wearing boots and stuff. IMG_2513.jpg