I have biothane, because of the upkeep. Leather needs to be oil now and again and need to be in a temp. Controled room if not used alot to keep it from molding. Biothane spray it down with 409 and and wash it with the waterhose. I love the look and feel of leather, but the care of biothane is easier.
I like the collar harness just cause I'm a ole farm girl that likes the look, but if your using a cart you can get a breast collar alot cheaper. Here's a website that has cheaper harnesses leather and nylon, don't know anything about them just what you see. www.ronshorseharness.com
If you go with a breast collar you don't have the added expense of buying a driving collar, the breast collar comes with it.