For clays a 28g No 7.5 is the most common. This is because the competition rules set it as the maximum load for some disciplines and most guys tend to stick with that. Also they tend to be reasonably priced. As for brand it is a matter of choice.

For the game birds I think No 6 30g or 32g seems to be fairly common. I dont do much game shooting so I could be wrong about that.

As for chokes there are two schools of thought. The ultra-scientific types that change constantly and the other extreme that never swap them out. I tried various combinations at the start but have half on the bottom barrell and three-quarters on the top barrell at the moment and havent changed them in ages. Some would argue that I am not good enough yet to be worrying about the subtle differences the choke makes and they may have a point.