I'm interested in trying the shotput because it seems like fun, not in competition of course but just to do something different. I've noticed shot puts come in a variety of materials like iron, cast iron, brass, steel, stainless steel, poly whatever, etc. The stainless steel and brass tend to be the most expensive but I guessing also the lowest maintenance due to their rust resistance.
Can somebody please advise which material they prefer for outdoor use and why?
Just go with the cheapest used ones you can get. No need to make expensive investments in something you might not like or do for very long. This way you can get 3 or 4 unless you like walking a lot, this also helps since you can redo your form a few times before you go and retrieve and lose all muscle memory of your previous throws. When your done using them bring them inside, clean and dry them off and they wont rust.