I use both, Brass is really the best without a doubt for Peg and Fence Snaring but I had a spate of using Steel Wire which is cheap ,effective ,, quick to make and fast to set against it is , you only get one catch and you have to replace the Wire , its too Strong [you may catch and hold something you don't want , you cannot knot it a bend is forever and you have to use a short Tealer to keep the wire stable and adjust the Loop so to me STEEL WIRE as its uses but for Rabbits it is Brass all the way
On the US sites a lot of States stipulate what Metal and Noose size can be used f