I've done lots of trap, skeet, sporting clays. It's all good fun, and 12ga. is pretty standard for it, usually with 7-8 birdshot, unless they require you to use steel shot, which more and more places are.
It's all lots of fun. I like the sporting clays courses best, when you're standing there and you don't know if it's gonna be a flyer, or crossing, or a roller. It is really gets the anticipation going and is a good challenge of you speed and reflexes.